Why did religion cause disagreements between Ireland and British?

The Brits are Protestant and the Irish are Catholic. The British formed the Anglican Church of England and especially when Cromwell invaded Ireland and attempted a genocide of the Catholic Irish, these religious differences caused problems that have lasted between the two. The British took all of the land of the Catholics in Ireland and gave it to Protestants, which you can still see today (its called Northern Ireland, which is part of the UK).