How can we know the IRA “won”? The country changed a hell of a lot over the course of the Troubles and by the time of the GFA in 1998 I don’t see how it is so clear that the reforms wouldn’t have also been achieved via other peaceful and democratic means.
What makes it giving up sovereignty? I understand it creates the potential for separation in future but not yet. The devolution of Scotland and Wales happened peacefully a couple(?) of years later, and Scotland may also separate in future.