I don't think so. Because the German Navy suffered such heavy losses to the Royal Navy in the Invasion of Norway in mid 1940, the Nazis never again had the power to contest the Allies at sea. To somewhat circumvent this, the Blitz was meant to gain air superiority over a potential invasion of England over the English Channel. Operation Sea Lion, however, was postponed on September 15, 1940 when the Luftwaffe incurred a particularly heavy defeat at the hands of the Royal Air Force. By October, Operation Sea Lion was called off completely by Hitler. So as to a potential invasion of Ireland, The Luftwaffe could not support any potential sea landing because it would be outside of their range and the Nazis also did not have a Navy that could contest the Royal Navy. One of the United Kingdom's greatest strengths in World War II turned out to be its island nation that the Wehrmacht could not take advantage of.

Zeiler, Thomas. "Annihilation A Global Military History of World War II". Oxford University Press, 2011.

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Well, the thing is, you could almost as easily ask why the British didn't invade! Irish neutrality during the war is interesting, to say the least! /u/Nonstr isn't wrong about the logistical shortcomings of any attempt to invade. Although Hitler commented on how wonderful a base Ireland would offer for bombing the UK, if Germany couldn't manage to cross over to the English coast, Ireland would be considerably harder, even given their meager defenses. Operation Green, the planned invasion of Ireland, even existed on paper, but importantly, it was only envisioned in conjunction with Operation Sealion at best (it was hardly a necessary component and wouldn't necessarily have happened even if they did invade the UK), not as a standalone matter. So the very short answer is that German consideration of an invasion of Ireland was at best serious only insofar as an invasion of Britain was, and when that didn't materialize, the invasion of Ireland became a pipe-dream at best. But just because the chance of invasion was slim doesn't mean that we don't have a ton to talk about!

Prior to the war, as war looked more and more likely, the Irish were willing to throw their weight around, comparatively. In 1938, PM de Valera insinuated that if Britain went to war with Germany while still still occupying Irish territory, it would engender hostility from the Irish, and as such as able to secure the British removing themselves from the Treaty Ports, three ports in Ireland which the British still occupied, and would have used to station escort destroyers had war broke out. And after that concession, the Irish only continued to press, now focusing on the partition of Ireland and Northern Ireland. As such, when war broke out, relations between the UK and Ireland were not their best.

The Irish Defense Forces had 6,000 active men in the army, almost doubled with reservists and Local security services when war broke out, as well as a few poor quality planes, and no navy at all. Initially, the Irish pushed a hard neutral stance. They did nothing to hamper the status of German nationals, and rebuffed British attempts to gain transit rights, let alone return use of the Treaty Ports as many expected. Churchill, being then in charge of the Admiralty, was livid, privately was accusing the Irish of provisioning German U-Boats, and trying to push for the taking of the Treaty Ports by arms. It didn't happen though, and although the ports did not return to British control, secret negotiations did result on concessions towards clandestine cooperation. British officers worked with the Irish for a joint defense plan against German invasion, and Irish coast-watchers, although not linking directly to the British, would transmit reports in clear, rather than code, purposefully allowing the British to easily intercept and use any spottings of German ships and subs. There was also unspoken approval to allow British "hot pursuit" into Irish waters. Churchill still wasn't happy, but his armed trawlers patrolling the Irish coast never spotted a single German sub reprovisioning...

Churchill's rise to PM of course did nothing to assuage Irish concerns about the British, and negotiations with both sides were going on, demanding Germany promise to respect Irish soil, and that if Germany won they would respect an independent and united Ireland (Which Germany happily hinted at), while at the same time seeking promise from the UK that they would rush troops to the defense if Germany invaded - but of course not a moment before a German boot hit the shore (and hinting that were they given Northern Ireland, maybe they would even join the war, which while tentatively explored, never resulted in anything).

British concerns reached a boiling point in November, 1940. Churchill gave a firey speech about how Ireland refusing to cooperate hurt British interests before Parliament, and in response, while the Irish 1st Division remained guarding the coast, the 2nd Division now began guarding the northern border (although it was more a political message of defiance than any actual fear the British were going to invade). The Germans, seeing an opening, offered to ship military equipment to bolster Ireland's meager forces, and in a perhaps not so subtle message, the equipment offered was British made arms that had been recovered at Dunkirk... Additionally, an "expanded military mission" was going to be seconded to the Embassy in Dublin. Realizing that while he wanted to be prickley, this would be an insult too far for the British, de Valera did not accept the German offer, and even ordered that if the German military mission flying into Dublin on Dec. 24 landed, they were to be arrested. So only a month after trading barbs with Britain, it was now the Germans who the Irish were faced off against, and the military was put on alert ("Which side?" was the supposed question of the Chief of Staff upon being informed). A plane did in fact show up, but never tried to land.

The Germans were pissed, and the German Ambassador alluded to consequences. A week later, bombs fell on Ireland two nights in a row, killing three and wounding twenty-four. Ambassador Hempel probably did not know this was what would happen, and on his counsel, nothing further happened, and the Germans in fact withdrew their offer of arms, as well as their general campaign to force Ireland into their camp.

As for the British though, Churchill was willing to keep playing hardball, proposing that the "400,000 tons of feeding-stuffs and fertilizer" shipped to Ireland could no longer be undertaken. Although part of a trade where wheat and fertilizer was exchanged for beef and beer (seriously), Churchill believed that the UK could do fine without the Irish side of things. Simply put, the pressure didn't work. The Irish were able to put up with some hardship, and make up for others, including imports from other neutrals like Portugal and Spain. And they could always point out that while the British civilian was rationed heavily on meats, butter and eggs, they were able to enjoy theirs with minimal trouble.

Now, there is an irony here. The Irish were neutral, and as noted, not trusting the British, but in the end, it would be wrong not to say that they were, in the end, at least slightly favoring of the Allies, if only for their own ends. They continued to try and expand their military, by 1941 with 40,000 men under arms and five times that as reserves, so arms and equipment was needed, and the UK and US obliged, including torpedo boats (essentially their entire navy), a few planes, and rifles. Even as they were negotiating for the arms though, namely a large shipment of rifles from the US in early 1941, Minister Frank Aiken bluntly (to FDR's consternation) asserted that they were to repel aggression, whether British or German.

By 1942, Ireland seemed less important. The US was in the war, and the benefits of bringing Ireland in as an ally, or at least a leaning non-belligerent, became less and less important. Sure, there was still attempts to undermine de Valera, but they were not as harsh or public as in 1939-1941. There was still distrust, but on several key matters, there was understanding and cooperation anyways. Allied and Irish counterintelligence officers worked jointly to monitor German military personnel in the Dublin Embassy. The last serious "attack" on the Irish came from US Ambassador David Gray and British Ambassador John Maffey, although it was hardly necessary at this point and speaks more to settling a score for earlier slights. Despite the aforementioned cooperation, which they of course knew about, they wanted to publicly embarrass de Valera's government, sending him both "notes" in Feb. 1944 requesting the expulsion of all Axis diplomats.

The Irish were of course seriously offended by what he took as a threat and undermining of sovereignty, and once again the Defense Forces were put on high alert. Rumors of invasion soon came up, and de Valera did little to quell them in his speech a week later, noting "at any moment war may come upon us", only feeding concerns. Instead of embarrassing de Valera by either a) Forcing him to comply or b) Publicly state he was "hostile to Allied interests", he had called their bluff, and now it was the Anglo-Americans who were embarrassed, since they certainly weren't going to follow through and do anything. Churchill of course, never one to back down, responded by curtailing exports to Ireland further, which just looked like more bullying and cooler heads had to issue a statement assuring the Irish that it was a temporary measure as the Allies built up their forces for the invasion and would be ended as soon as possible.

De Valera's popularity only went higher and higher, having stood up against the Brits and not flinched. And then he almost screwed it up! Having shepherded Ireland through the way safely, on May 2nd, he personally visited the Irish embassy to offer his condolences for the death of Hitler... The Irish people may have enjoyed seeing him pull the British lion's tail, but they certainly didn't like that, just when the World was learning of the true extent of Nazi horror. Churchill of course could not leave it unremarked, noting in a May 13th speech that the Irish "frolicked with the Germans and later the Japanese representatives to their heart's content". Had Churchill just let de Valera's words stand, maybe he would have finally won the "victory" he craved, but this simply opened a window for de Valera to respond, giving a stirring radio address, both reminding the people of Britain's hostility, but also praising Britain's victory where she stood alone against Germany for a time, comparing it to a similar, nearby island which also 'never accepted defeat nor surrendered her soul'. The Irish ate it up, and his misstep was soon forgotten. (EDIT: Link to the speech, thanks /u/International_KB)

So, how "serious" was Operation Green, then? I mean, the whole job of a General Staff is to come up with contingency plans for pretty much everything, right? So was there ever real consideration given to doing it, was it more a theoretical exercise?

"Serious" is difficult to quantify. Were mobilization plans ever executed upon? No. Were there German agents operating in Ireland and some studies a la "Operation Green" type research? Yes.

As you touch on in your question... Ireland was considered an appetizing strategic position for the Germans against the British, as it would open up a western flank against Great Britain, and provide a superior base of operations for naval support and aerial patrols in the North Atlantic. That's not to mention the anti-British sentiment among a great number of Irish citizens (not the majority, but enough to provide a significant native support network).

As far as exploration of a real plan, the IRA did share intelligence with the Abwher. Mark Hull wrote a fantastic book on the subject; Irish Secrets and German Espionage in Wartime Ireland. But Germans were more afraid of a counter-offensive - knowing the British could quickly counter-invade, and probably occupy / control / defend Ireland far easier than they could.

Along the same lines, Britain actually wanted Ireland to join the war on their side - going as far as offering Northern Ireland be united with the Irish Republic if there was an Irish Declaration of War against Germany and Italy. de Valera rejected this, there's not a lot of great sources on the matter, but my understanding is that he feared the offer was not genuine - and Ireland would need to commit troops to a conflict while the unification would not be clearly part of the deal until after the war.

On the German side, many officials resisted the idea because any slight step towards an alliance with or an invasion of Ireland would justify the British to do the same. Much as with their hope for the U.S. - the Germans saw an alliance as unlikely and problematic for a variety of logistical reasons, and felt neutrality was the best possible outcome for Germany.

Would Ireland's neutrality have worked as a deterrent of sorts? Also, was there ever any sort of evidence of the Germans trying to get the Irish on their side?

As a follow up question: I always heard that Ireland was moderately supportive of Germany, providing some volunteers and resources. Is that just an urban legend?

Kind of unrelated, but how did the nazi racial ideology view the Irish as a race? I've seen a few examples of "racial science" from the late 19th century that tried to present the Irish as sub-humans. I'm interested in whether or not the Nazis incorporated this kind of thinking into its ideology

Naval invasions are incredibly hard.

Think about D Day: the allies had complete air control, complete naval control, and the vast majority of german troops (including the best forces) were involved on they Eastern front. Still it was considered a risk.

Now consider a situation where the English would have control of the airspace (because of the large distance for the German airforce), near complete control of the seas, and the ability to reinforce Ireland MUCH easier than Germany. If the German's somehow managed to have a force land, then it would be: Cutoff by a superior British navy, without supplies, under constant aerial attack, without heavy armor, and without retreat. Even in a best case scenario, the German's would have not been able to reinforce any footholds they got.

That Operation Sealion would have failed is pretty well established. Given that an invasion of Ireland would have been far, far harder to achieve than a cross Channel invasion, it seems unlikely that it could ever have been seriously contemplated. That it was an invasion of neutral Ireland wouldn't have stopped the RAF and the RN from attacking the German invasion fleet and interdicting the supply train. In any case, what would they have achieved by invading Ireland?

I'm not going to add an elaborate answer but I'll offer some clarification and sources. It seems that German invasion plans were always in the context of an invasion of Britain rather than opening a new front. The Irish did discuss the possibility with the British government and agreed that they would call for British assistance after the Germans attacked. An IRA proposal for a German invasion supported by them (codenamed 'Kathleen') does not seem to have been taken seriously by the Germans. The Irish took the view that open collaboration between the IRA and the Nazis would be positive as it would allow Ireland to finish off the IRA. As it was Éire interned IRA suspects and allowed hunger strikers to die.

Irish neutrality was motivated by a desire for national unity. The government was afraid that an open pro-British policy would meet popular opposition and split the country as it had in 1914-18. Remember that De Valera and many of his government owed their political careers to those splits. To British politicians it seemed unthinkable that a commonwealth country could be neutral and this was one reason they reacted so much more strongly to Ireland than to other small nations. One reason why Britain never attacked Ireland was that Northern Ireland provided them with many of the same advantages.

There is no doubt among historians that the Irish government wanted an Allied victory but public opinion was more divided. It is hard to judge this accurately as censorship forbade the publication of anything favouring either side. This even extended to reports of Catholic Bishops' pastoral. There is some evidence of pro-German sentiment and much more of anti-British views.

A note on the internment of Allied forces. The position was that service men on operations were to be interned but not those on training missions. Very quickly it was assumed that Allied personnel were the latter and Germans the former.

Other assistance to the Allies such as weather information and intelligence sharing are well documented but were kept from the public. The government also made it very easy for Irish people to travel to join the British army which thousands did. The whole policy of neutrality was as much propaganda as reality. Unfortunately De Valera took the pretence too far in signing the book of condolences for Hitler. This was an act of crass stupidity which was opposed by his ministers and damaged Ireland's reputation for decades.

There are two interesting historiography articles on the subject ( JSTOR required )

Ronan Fanning, 'Irish Neutrality: An Historical Review', Irish Studies in International Affairs Vol. 1, No. 3 (1982), pp. 27-38 http://www.jstor.org/stable/30001722

White, Timothy J., and Andrew J. Riley. "Irish Neutrality in World War II: A Review Essay." Irish Studies in International Affairs (2008): 143-150. http://www.jstor.org/stable/25469840?seq=1#page_scan_tab_contents

A very large number of documents from the Irish Department of Foreign Affairs have been published and digitised. They can be accessed here http://www.difp.ie/

The memo by Viscount Cranborne detailing Ireland's pro-British activities during the war is available at the British national archives http://discovery.nationalarchives.gov.uk/details/r/C9094627