Fly's 4 options are all plausible but each poses problems.
Opinion poll results will reflect the bias of the pollster's client so are likely to be volatile.
Declared faith isn't directly matched to voting. And recent evidence (EG Abortion Referendum) suggests people are giving up faith in large number. If you're simply using Catholic as shorthand for Nationalist, why not count the Nat vote in an election or 2. They're still pretty frequent despite loss of Stormont...