Why this fascination with making Snape Catholic when he's four times more likely to be Anglican than Catholic?
I'll let bohemianspirit answer for herself, but as somebody who's written Catholic!Snape, and who always views him as such, I can give you my reasons for doing so. In my case, it's not random, and it's done with the full understanding that Catholicism is far from the dominant religion in the UK.
If we were considering any other character in the novel, I'd agree with you: not likely to be Catholic. Hermione? Obviously C of E. McGonagall? Church of Scotland all the way. And do the Weasleys even have a religion?
Snape, however, has several factors that tilt the odds more in favor of Catholicism. His mother's very Irish last name might be one clue (though hardly definitive), and there's also the fact that he's Northern (not Scottish, we think) and very working-class. These are just very circumstantial things, but I think they make it at least a bit more likely. One also might consider his guilt issues, and even his priestly black robes.
What really sways me in favor of Catholic!Snape is his personality and his outlook on life, which I think strongly demonstrate very Catholic attitudes. An emphasis on sin, confession and earned redemption are important, and it's key that Snape's emphasis is on the performing of good acts, rather than 'nice' words. Catholics believe that grace is earned through the performance of good works, not simply granted (that's Protestant), and I think that comes through very clearly in Snape's deeds and also in his frustrations.
Sorry for the tl;dr, but that's just my take on it. I know there was a good conversation about this topic on schemingreader's journal a while ago, too.