My family is Irish and Catholic. I am the only one who is not really a big fan of ND, although I do respect Coach Weis a lot. Sister and brother-in-law both went to ND, and father is sickeningly obsessed with them.
Having gone to a few games with them (lots of ND-Navy games, seems I was always pulling for the underdog Middies), I can tell you there fan base is largely white Catholics. They go to mass en mass, and you wouldn't believe the amount of private Catholic schools there are in some parts of the country. Just about every kid wants to go to ND.