Black Tom Cassidy, a recurring villain from the X-Men comics, is a lapsed Catholic. There are sufficient textual references from the comics to ascertain this much, but we have not yet catalogued all such information on this page.
Not sure about the Cassidys, given that high-born Irish families have changed religion more than once over the centuries for various reasons to do with land inheritance. The only lead I've found is that Tom sent Theresa to a Catholic Boarding school. I've been going off that...
From: "Who's Catholic in the Marvel Universe" forum discussion started 5 February 2005 on "HCRealms" website (http://www.hcrealms.com/forum/showthread.php?t=123637; viewed 10 May 2007):
I know a lot of characters are Jewish, so I was wondering who is officially Catholic?
I know Daredevil is. It's a major part of his personality and often occurs in storylines.
I also believe Firebird from the West Coast Avengers... After that, I'm pretty much stumped.
Anyone have any others?
How about the Irish contingent in the X-Universe:
Black Tom Cassidy