As already stated, the Irish government's policy was that the IRA were terrorists and their violence was counter productive. In the early stages of the Troubles, this caused a significant split in the ruling Fianna Fáil party. The Taoiseach (Prime Minister) Jack Lynch and the Minister for Foreign Affairs, Patrick Hillary favoured support for Northern Nationalists by non-violence and diplomacy. A more radical faction of the Cabinet associated with Niall Blaney wanted to actively support the IRA in the hope of achieving Irish unity.

In April 1970, the Gardaí (Irish police) seized arms intended for Northern Nationalists. In May Lynch sacked Blaney and the Minister for Finance, Charles Haughey, and they were charged with smuggling the arms. When they were acquitted, it looked as if Lynch might fall but his side prevailed and his policy became the standard for the remainder of the conflict. When Haughey became Taoiseach in 1979 many expected a change but while his rhetoric was more green, his policy was not really.

The Irish government brought in legislation allowing the detention of IRA suspects and special courts to try them. Sinn Féin and other parties with terrorist links could not be interviewed or quoted on Irish radio or TV but were not banned.

The Irish government sought to bolster constitutional nationalism, especially the SDLP which was the largest nationalist party. They supported the power sharing assembly in the Sunningdale agreement for this reason. Likewise the Anglo-Irish Agreement was designed to help the SDLP after Sinn Féin had some electoral success after the hunger strikes. This policy only changed in the early 1990s when back channels were opened by the IRA to discuss ending the conflict. In fact there is evidence that the Irish were more suspicious of the IRA's intentions than the British.

As for your second question, it is very hard to generalise. Most Northern Nationalists held Irish citizenship and saw themselves as Irish but there was a strong sense that the South had not stood up for them in the past and needed to do so now. Most seemed to support the SDLP view that the Irish government should be the nationalists' ally in negotiations with Britain and did not see the governments as neutral. The SDLP wanted cross border bodies with real power as part of any solution. Nationalists rejected the idea of an internal solution without Irish involvement.

In addition to the below, its worth noting that quite a few Garda were murdered by various nationalist entities.

The first Garda to be killed in the troubles was Richard Fallon, shot by a group called Saor Eire.

Michael Clerkin was killed when the IRA lured the Gardai to a farmhouse which had been booby trapped with an IED. Several other Garda were injured.

Further examples include the INLA shooting dead Henry Byrne and John Morley following a bank robbery in 1980, and they also shot dead Patrick Reynolds also apparently as result of a bank robbery.

The whole thing is less than clear cut though. Just as there are allegation of British security forces colluding with unionist paramilitaries, there are also allegation that the Gardai colluded with nationalist paramilitaries.

The IRA was, and is, regarded as a terrorist organisation in the Irish republic, and is, as such, an illegal organisation, membership being a criminal offence since 1935. Indeed, the republic had executed many IRA members. Hence the relationship would have been secret, through intelligence channels only, if any relationship existed. Incidentally, the IRA had been supported and in part financed and trained by the Nazis, and by the USSR, many members being Marxist in ideology, certainly by the 1950's.