Interestingly enough the Scots invaded Ireland in 1315 after their victory at the battle of Bannockburn in 1314. The campaign was led by King Robert the Bruce's younger brother, Edward, and the reasons for it are debated. One is that it opened up another front with England in the First War of Independence, as the Scots were on a militaristic roll and it was a way of pressing this advantage with a grand foreign campaign. Secondly it got rid of Edward, quite permanently when he died in battle in Ireland in 1318. The prince, like all the Bruce's, was an ambitious character and it is possible King Robert and his advisors wanted him away playing king somewhere else. Historians in the past, like Evan Barron, emphasised the idea of a pan-Celtic unity that made the campaign in Ireland appealing to the Scots. However, this is hotly debated and the concept of any kind of national racial identity before the Modern Era is likely to cause arguments. Sean Duffy wrote a book on the subject of the Bruce's war in Ireland and you'll find some mentions of it on works about the First War of Independence, Barrow for instance, but it remains a relatively unexplored area of study! I hope this answered your question in a way, and if you want me to elaborate in any way feel free to ask!

For Ireland there was only ever really Britain

That’s not exactly true. The Vikings set up a Kingdom centered around Dublin in the 9th century, and had several other colonies across Ireland including Wexford, Waterford, and Cork. But I imagine this small scale settlement isn’t what you imagined when you asked about invasions of Ireland.

As others have said, yes the Spanish and the French at various times landed relatively small bands of troops in Ireland, usually with the hope that they’d inspire a Catholic rebellion against the system of English control over the island. These expeditions were few and far between, largely due to the threat the Royal Navy posed to large landing forces and the supply convoys that would necessarily follow. The English and their Irish lords were sensitive to possible rebellions as well, and the addition of a few thousand continental troops was insufficient to pose a serious threat.

Besides the Royal Navy, Ireland is a geographically isolated island. So long as the British wanted to keep the Irish under their dominion, it would be very difficult for anyone else to stop them. Even if someone had beat the Royal Navy on the high seas and managed to conquer Ireland, the new Irish state would be under constant threat from an English counterattack across the narrow Irish Sea. It simply wasn’t worth the effort to (1) beat the Royal Navy, (2) amass a large enough army to subdue Ireland, and (3) Garrison the island in perpetuity against further English attacks or internal rebellion.

If you look at Ireland on a map you'll see that it's basically penned in by Britain and very difficult to reach from the continent without moving through British waters. As such almost all dealings in Irish history with foreign powers come from Britain since it's the path of least resistance (apart from the Vikings).

On many occasions the enemies of England did recognize that Ireland was a potential 'Back door' to damage the English, the most important examples of this were the Nine Years War (the Spanish landed in Kinsale as an extension of Anglo-Spanish war in support of Hugh O'Neill), the Irish Rebellion of 1641 and subsequent Confederate wars until the Cromwellian reconquest (a Papal Nuncio Giovanni Battista Rinuccini was sent with large amounts of money to support the Confederates and became massively important due to his resources and position as a direct representative of the pope), the Williamite War (massive amounts of continental troops were sent to Ireland, especially by France, to support James the Second against William the Third, this was by far the largest amount of continental troops sent to Ireland while the country was nominally under English control, they were defeated at the battles of the Boyne and Aughrim), the 1798 rebellion (Revolutionary France tried to support the United Irishmen's revolt in the country, landing troops in the west after the brief but bloody rebellion had already been defeated and quickly being defeated themselves) and the 1916 rising (the Germans sent a ship carrying weapons to try and assist the rebellion but the ship was discovered, intercepted and scuttled by the captain before they could help the Irish).

A reoccurring problem with the French or Spanish attempts to invade Ireland was the difficulty in landing troops on the island, the Spanish lost their second Armada in 1596 trying to get to Ireland with it being decimated by storms and could not reach it, the same happened to a large French invasion force during the disastrous Hoche expedition 200 years later in 1796. Both forces were much larger than the ones that ultimately did land in Ireland some years later and probably could have massively changed the course of the wars that were underway, alas, the difficulty of approaching Ireland for a continental power that needed to keep a wide berth of British naval power in the most easily reachable locations was considerable, and the weather ruined these attempts.

Finally, you have to keep in mind that, quite honestly, Ireland's not that useful strategically or economically to control. The country has almost always been poor and underdeveloped without many resources which makes it a lot less attractive to conquer compared to somewhere like the Netherlands. It's main strategic use is specifically as a way to cause trouble for the English and a potential place to stir up rebellions that could distract them from affairs on the continent (in the leadup to WW1 many in the German leadership were convinced that an ongoing crisis over home rule in Ireland might be enough to distract the British from taking part in WW1, obviously this didn't happen but it might have influenced German willingness to invade Belgium), but outside of that it's hard to reach, being a north Atlantic island on the very edge of Europe and offers little in return. Meanwhile the English had a very strong vested interest in maintain firm control of the island after sea power became more important in world affairs to prevent themselves from being made vulnerable to their immediate west.

An incomplete list of invasions of Ireland:

- Viking invasions (795-900s)

- Anglo Norman invasion (Strongbow et al.), followed by Henry II (1169-1175)

- Edward de Bruce of Scotland (1315)

- Tudor Plantations/Tudor Conquest (1556-1603)

- Spanish invasion (1602)

- Cromwellian invasion/war of the three kingdoms (1641)

- Williamite war (1690) *

- Attempted invasion by French Republic in support of United Irishmen, United Irish Rebellion, the Year of the French (1796, 1798)

*Arguably a Dutch invasion, though one might need to keep tongue firmly in cheek here.