Ireland's Catholic-Protestant conflict rests on multiple, overlapping differences: religious, ethnic, colonial, political. To better understand it, and in particular its religious aspect, I trace Europe's long Catholic-Protestant conflict, how it began, reproduced itself over time, and finally came to an end in the twentieth century. I then use this to generate insights into Ireland's long conflict, how it began, how it developed, why it ended in the Republic, why it has continued in Northern Ireland, and how the latter conflict might end.