I fairly agree with you. It doesn't matter if Seamus is from Ireland or from Northern Ireland (people use to forget there are differences!), religion was probably a part of his life in some way, maybe he's not even Catholic (or protestant), but he'd know about religion and probably influence his life. Living in a Catholic country (which doesn't mean everybody is Catholic, of course), we do have a relationship with religion: festivities, cursing, sayings... all that is related, and it's a part of our tradition (even though some people is starting to deny it). It's also part of our culture: whether in paintings, Cathedrals or Castles.

I can understand Harry not being religious, since the Dursleys don't seem religious at all, and he seems to have been pretty removed from the "outside world".

I think Rowling has purposely avoided the issue of religion in her books, but what we do with fanfiction is to explore other aspects of the books, and religion is an interesting angle to look at them.

you can tell (as a Catholic) that the author isn't Catholic and barely did any research other than what's in the movie or available on the wiki.

Let's face it, authors in general don't bother to research about what they are writing. Very few of them post questions at hp_britglish (for example) and I could tell you about the inaccurate Spanish or Spain related things I've read in fics (and what's worse in published and awarded books such as "Bel Canto").