Every incarnation of the IRA has declared the Irish state to be illegitimate, so unification, absent a Sinn Féin takeover down South (highly unlikely at any point in the late-20th Century), is totally off the table. Putting aside the implausibility of the British being forced out in the first place, this just isn't Vietnam, and the nationalists have every reason to expect marginalisation in a post-withdrawal Northern Ireland. The loyalists, meanwhile, now have virtual carte blanche to settle scores with the Catholic community. Short of an intervention by the Republic (an effort that would necessarily preclude the annexation of Catholic-majority counties), things are going to get extremely ugly.