I take massive exception to this. Catholicism is so ingrained to into Irish nationalist culture that it was dangerous for me to say in school that my grandmother is Presybterian. Catholicism IS the culture.
Similarly, I've a couple of friends from the North who are of various Protestant dominations, and I've seen them face outright hostility for mentioning it.
And on the flip side, whenever I'd go up North as a kid or a teenager it was always made abundantly clear to make sure NO ONE knew I was Catholic. It didn't matter if people knew I was from the Republic, but revealing I was Catholic was like the biggest trigger for trouble I could do.