I’m from Northern Ireland so I can (try to) explain. It’s a joke about entrenched tribalism here. No matter who you are or what your beliefs are, you’re judged on where you’re from (a “Catholic area” or “Protestant area”) and what your family’s religious background is.

For example, I’m considered to be a part of the Protestant/Unionist community due to where I grew up and who my family were even though I am completely non-religious and my worldview has no resemblance to that of traditional Unionism. I’m descended from British people who colonised the North of Ireland during the Ulster plantations.

The joke is just exaggerating and poking fun at the divide, implying that you could be literally anyone and would still be asked if you’re Catholic or Protestant. I.e. “I’m Muslim” “Yes but are you a Catholic Muslim or a Protestant Muslim”.

Hope this makes sense. Like pretty much everything with the past 5 centuries of Irish history, its hard to explain.

The nerdy girl in the movie actually being stunning IRL was always amusing