Éamon de Valera, who was arguably the most powerful politician in Ireland for its first few decades of independence, was a strong proponent of Irish reunification; as late as 1945, in a speech about Irish neutrality during the just-completed war, he compares continued British "holding" of the Six Counties with German "occupation" during the war, and uses this as part of his justification of neutrality. And even the anti-treaty politicians accepted partition as a necessary evil rather than a matter of democratic self-determination for the Northerners.
In 1998, on the other hand, an overwhelming majority of Irish citizens supported giving up irredentist claims and fully adopting the idea that the people of Northern Ireland, rather than those of Ireland as a whole, should decide that region's political future.
So when did this sea-change come about? Was it sudden or gradual? What were the main causes? Or, alternately, was there never any sea-change and I'm misinterpreting the information here?
Note: I know this is a seriously touchy issue. I've tried to avoid language that will upset anyone, or state my own opinions at all in my statement of the question. But if I've messed up, feel free to send me a PM and I'll try to fix it.
Thing is, this is probably going to be difficult to answer without reference to the Good Friday Agreement and what led up to it, which you yourself have indirectly, and which does fall under the 20 year rule. Maybe you could try /r/AskSocialScience if you don't find someone who can answer in those constraints here?
There were several phases of North-South relations. We have to distinguish rhetoric from reality as southern statements were much more hard line than southern policy. There was a brief period of real irredentism when the south actually supplied the IRA, paid teachers who refused to accept the Northern government and encouraged local councils to declare allegiance to the south. This ended almost as soon as the Free State was formed in 1923 as it was financially and militarily unviable. The Boundary Commission was initially expected to resolve the border but when it finally met it only recommended minor revisions and its report was shelved. What followed under both Cumman na nGaedheal and Fianna Fail governments was a policy of rhetorical irredentism while actually accepting the border as real. Fianna Fail always did better at playing the ‘green card’ but did little in practical terms to change the situation. The southern state did not give representation to Northern nationalists in its institutions and it retained a customs border with the North. It also acted to suppress the IRA, using internment during WW2 and the Border Campaign of the 1950s. Southern policy focused on the British government rather than the Northern one. The actual articles 2 & 3 of the Irish Constitution of 1937 are a good example of this double policy. While they claimed Irish sovereignty over the north, they did not make any specific policy claims or offer any process for unity.
This policy began to shift post WW2 for a few reasons. The declaration of the Republic led to a hardening of the British position. In the 1949 Northern Ireland Act, Britain guaranteed that Northern Ireland would stay in the UK unless its parliament opted out. The Republic intensified its rhetorical opposition to partition but notably refused any real assistance to Northern nationalists. The result was an impasse. This began to shift in the 1960s under Sean Lemass who met with the Prime Minister of Northern Ireland Terence O’Neill to discuss economic co operation. Lemass had impeccable republican credentials and could get away with a policy of détente. This continued under his successor Jack Lynch but fell apart with the outbreak of the Troubles.
The troubles led to an initial resurgence of irredentism. Fianna Fail split between Lynch’s supporters who wanted to manage the conflict and those of Neil Blaney and Charles Haughey who saw an opportunity for unification. Lynch won, but at great political cost. From then on the policy of the Irish government was to support constitutional nationalists and promote a political solution while opposing the IRA and containing the conflict within the six counties. In 1973 the government of Liam Cosgrave signed up to the Sunningdale agreement which would set up a power sharing government in the North. The south agreed to accept the north in return for a cross border Council of Ireland. Unfortunately the executive collapsed under pressure from unionist strikes. Southern policy from then on was that in return for internal reforms the south was willing to compromise on its claims but that it would not do so unilaterally.
While these policy changes were taking place there had also been some developments public perception. While most people remained committed to unity in principle, many intellectuals had begun debating how to make it attractive to unionists. Most notably Conor Cruise O’Brien offered a revisionist critique of the Irish State which questioned its relationship with violence and Catholicism. This was not a popular view but it was challenging. The senior Fine Gael figure Garret Fitzgerald also began to put forward the idea of a more pluralist Ireland.
When Fitzgerald became Taoiseach his northern policy was to promote the constitutional SDLP against the IRA. He negotiated the Anglo-Irish Agreement with the British government. Under this the southern government would be formally consulted by the British in Northern Irish matters and would accept the so called principle of consent: IE that the majority in NI would decide its future. Documents from the negotiations show that FitzGerald was willing to hold a referendum to remove Articles 2 & 3 but the British decided not to press for this as they felt it would be defeated. The agreement was popular in the south but was initially opposed by Fianna Fail under Haughey. Once Haughey was elected in 1987 he reversed his position and used the agreement.
It is hard to judge public opinion but it is clear that while violence did not stop southern voters supporting the principle of a united Ireland, they saw peace as a higher priority and steadily accepted the consent principle articulated by John Hume the leader of the SDLP. To that extent the IRA campaign seems to have significantly reduced traditional irredentism in the south. The initial peace process up to 1995 (and 1998) including the Downing Street Declaration of 1993 and the 1994 IRA ceasefire was extremely popular. I won’t go beyond 1995 but there was little change in this view in the south.