Even if the social differences or the political parties aren't the crucial point, I would say that they still are an (important) factor. But I would agree with you that the ethnical differences are the main problem. And I would totally agree to what you wrote about the "catholic identity"! I think that's the best explanation for the role of the religion in the whole conflict .

I think we can't really compare the crusades to the members of the IRA, because the crusades fought against an other religion, the IRA-members fight against an other confession. I would agree that the religion wouldn't stop them, but I can not imagine that they would kill more people. The Protestants basically believe in the same God as the catholics, so that wouldn't work as a base for a religious war. It's all about politics ;)