Blimey, that's a hell of a question. There hasn't actually been any unilateral disarmament though.
Long story short- people got sick of it and eventually enough people on all three sides were willing to compromise on revenge to get peace.
The big problem with these things is that if they end, they always end with one side 'up', as it were. Terrorists bomb, the Government won't stop until it strikes back. Government strikes, terrorists won't stop until they have revenge. And so on, and it goes on forever.
Right now, that cycle has been broken in Northern Ireland, and enough people convinced to just live with it in the name of peace. The plan now is to keep that peace long enough so that no-one feels the urge to do any different any more.
Yeah, let's just leave it at saying that the total US contribution to the Northern Ireland situation has been unhelpful.
It was a Canadian General who helped oversee the process.
That's not to say the people involved were incapable of settling peace, just that obviously you need an external broker to moderate this kind of thing.