When I was at school( in England), I was unaware of the religion of my mates. We didn't have any expressions like "left footer". We didn't talk about religion. It might have been different if there had been a lot of ethnic minority pupils at my school, but there weren't.

I am aware of the football issue, but from what you say it is more to do with belligerence than with deep-seated sectarian antagonisms. The difference with England is that, as far as I am aware, no club is seen as Catholic or Protestant, even nominally.