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.

I mean a lot of atheists are still so impacted by the religious culture and refuse to let go of it, to the point where even the pope of the nu-atheism richard dawkins himself said he was "culturally christian" just to make sure that everyone knew he is a white supremacist at heart.

Source: https://m.youtube.com/watch?v=COHgEFUFWyg

No its about how cultural values are influenced if not heavily shaped by the dominant religion

It was nice knowing you

See that wall over there, bud?