Why are the Irish so hostile to the British people and Govt. ?What for are they agitating ?

I had read that during World War II, the Irish spies actively helped the German pilots in bombarding London by poiting out important targets.Except Cathilicism & Protestant faith are there other hles also ? Dont they speak the same language? What is Sinn Fein ?Why is Ulster, Munster seperate from the rest of the Irish island?