When the Irish constitution was amended after the 1998 Good Friday agreement to replace an apparent claim to jurisdiction over Northern Ireland by an aspirational statement, it seemed that many of the issues of conflict in the North–South relationship had been resolved. This article traces the process by which ideological change and policy shift in southern Ireland during the course of the twentieth century facilitated this agreement and the associated constitutional reformulation, looking at three areas within which change is obvious. First, demands for Irish unity, vigorously expressed but confined substantially to the domain of rhetoric, were softened in the early 1970s when the fuller implications of Irish unity became clearer, and in the context of a possible British withdrawal from Northern Ireland. Second, distaste for North–South institutions such as a Council of Ireland, on the ground that they implied recognition of partition, was replaced by acceptance of a modest level of institutionalised cross-border cooperation. Third, reluctance to recognise the legitimacy of Northern Ireland was reversed, with Irish governments moving progressively towards recognition of the principle of “consent” in the late twentieth century. Together, these changes amounted to a reversal of traditional irredentist policies and a formal acceptance of partition.