Prior to the Good Friday Agreement in 1998 parts of the Republic of Ireland's Constitution claimed the island of Ireland as a whole national body, which obviously includes Northern Ireland.
What was both the political and civilian response and opinion to The Troubles and the subsequent civil rights movement in Northern Ireland? Was there any rhetoric from Irish politicians to unite the island under Irish rule? Were there any significant recruits from Ireland to the IRA in Northern Ireland?
Obviously opinion varied a lot during the Troubles. I'm going to distinguish between popular and political opinions but they were closely intertwined.
During the early crisis of 1969 there were demands for Irish intervention either on humanitarian grounds or even military action. The government resisted this as they were afraid it would spark an anti Catholic pogrom and because the Irish Defence Forces advised that they could not complete a mission inside Northern Ireland successfully. Arms were smuggled to republicans with the knowledge of at least two government ministers who were subsequently acquitted of the charges.
After British troops were deployed, Irish sympathy remained with the Catholic population. The introduction of internment and the massacre of protesters on Bloody Sunday in 1972 led to mass demonstrations and the burning of the British Embassy in Dublin. Despite this the government was consistently opposed to the IRA which it saw as a threat to the Republic. Overall the government wanted to contain the conflict and prevent it spilling over into the South.
The Irish public similarly seem to have distinguished between support for a united Ireland (which remained high) and for the IRA which was much lower. Although there were pockets of IRA support around the country the vast majority of Irish people North and South opposed the "armed struggle" in opinion polls and at elections. Individual events did cause surges of support such as the deaths of 10 republican prisoners on hunger strike which led to the election of 2 republican TDs (Irish members of parliament). This was not sustained and the IRA's political wing Sinn Féin averaged only about 2% in later elections before the ceasefires. During the course of the troubles, Irish support for Articles 2&3 actually fell.
There was political disagreement over Northern Ireland with Fianna Fáil opposing the Anglo-Irish Agreement while in opposition although they upheld it when back in government from 1987. There was a broad consensus that the Irish government should support the constitutional nationalists of the SDLP and the Catholic community in general.
As I mentioned earlier there were pockets of IRA support especially around the border. These provided finance, safe houses and intelligence. There is also evidence of collusion by some policemen. Historians have to be careful about this as sources are only emerging and Irish libel laws are extremely severe.
There is a contrary view to what I have written, most cogently argued by Conor Cruise O'Brien that the Irish public were unwilling to face the reality of IRA brutality and hid behind a false distinction between the 'old IRA' of 1919-21 and the 'Provos'. This was not a popular view but it did influence a number of journalists and historians. The opposite view, from the republican side, exaggerated popular support and blamed censorship and media bias for its failure to manifest itself.
There is no big source book for this subject. I am basing my account on a lot of reading over the years. There is an interesting article here:
Bernadette C. Hayes and Ian McAllister, 'British and Irish public opinion towards the Northern Ireland problem' in Irish Political Studies 1996.