Whee! Political debates that don't center around the US!

For those of us isolationists that don't understand "The Troubles", what is the deal? As far as I understand it, Ireland was invaded by Britain ages ago and there was an Irish revolution that eventually resulted in an independent Ireland. But Northern Ireland was kept from that nation and remained a part of Britain. That brings up question #1: why? Were there more British in that area than elsewhere? So since then, some have campaigned for the "return" of Northern Ireland to Ireland, and others have campaigned for Northern Ireland to remain British. Question #2: why? I think I understand the last part, which is that it got all heated and violent and escalated from there until it's probably gotten to the point of being a feud with mostly-forgotten reasons, but I just don't understand what brought it to that point to begin with.