oh gosh expat...basically the English invaded Ireland a fairly long time ago (about 400 years?). Then they moved a lot of Protestant Scottish people there, mainly in the north, in the usual British Establishment tactic of divide and control, ie they would keep the Catholic Irish busy by fighting with them. It worked, and is still working today. Traditionally, I believe it is true to say that the Protestants had the support of the British government, and they got the best of whatever was going in the past.
When the English had to give up the rest of Ireland after WW1, they conceived the bright idea of splitting 6 of the 8 Ulster counties off and keeping it, thus keeping the Protestants in the majority in this small area. Otherwise, of course, they would have been in the minority in the whole of Ireland.
there are a few other details, and someone may come along and tell me that Ive got it all wrong, and it is actually quite different.
Why certain people in Northern Ireland want to keep these centuries old fights going is a total mystery to anyone not born in NI, or sadly, in parts of Scotland too. The only logical explanation is that someone, somewhere, is making money out of it.
An Irish girl once told me that she and her hubby met some Northern Irish Protestants on holiday in another country. They got on really well, but at one point, the Northern Irish said "Of course, if we were at home, we wouldn't be able to talk to you, you know..."