This is fantasy, any atrocity on that scale would have the opposite effect there is no way that Thatcher would be seen to give in to terriorists. Also given the various predictions of unionist resistance, I am not sure that Patrick Hilary would have wanted Northern Ireland and certainly not in the aftermath of a terrorist outrage. Effectively the IRA would be able to claim credit for unification.
A lot of people might have wanted to pull out of Northern Ireland but not in the aftermath of an atrocity likewise reunification from the Southern perspective. Reagan regarded Thatcher as a good ally and Britain as a useful aircraft carrier so he would hardly lean on Thatcher