I attended a Catholic school up to the 7th grade. I had Jews and protestants in in most of my classes that I can remember.
In Northern Ireland, a Catholic school isn’t the same as a Catholic school in the USA. It isn’t a parochial school where nuns walk around. It’s just a regular school in a certain part of town.
There are Catholic pubs, Catholic stores, Catholic playgrounds, etc. And the Protestants have their own facilities as well.
People are separated more by a cultural identity than a religious fervor.