beanaseireann I saw your post last night and I've been trying to decide how to answer, but I can't! Obviously some things are instant giveaways - it's very unlikely that someone called Saoirse would be from a protestant/unionist background! Other than that... I can't quite put my finger on it tbh. The closest that I can come, is that people from Catholic backgrounds feel more familiar to me as an Irish person if that makes any sense? If you've ever seen the threads on here comparing Irish and British culture, manners etc it would be similar to that, with Catholics coding as Irish and Protestants as British. Only even more subtle as obviously people have far more in common than not.
It is subtle though... Obviously unless certain topics of conversation come up.
Also, people tend to feel a bit awkward about the whole thing ime, and try to be sensitive to people from another background. For example, I've noticed that naice middle class protestants trip over themselves trying to get Irish names right - a colleague I'd known and sat next to for years one day said "can I ask you something" in a way that I thought meant he was going to ask something very personal. It was, "how do you pronounce Eimear"! He'd guessed right, and even used to know someone with the name, but a) didn't want to get it wrong with a client and offend her and b) wanted to be so sure he didn't offend me by asking. He completely over thought it. Similarly, my DD has an Irish name that's almost phonetic in English, and I don't care if someone gets it a bit wrong. My GP tried so so hard, so earnestly to get it right he called her a completely different name.
But he just wanted to be so very clear that he didn't care we're (culturally) Catholic in our largely Protestant area.
Similarly, I've been in the same scenario twice at work (different jobs) where a bunch of women have been having lunch and discussing school. In both cases, all bar one or two were from Catholic backgrounds and so several school mass stories were told. Afterwards, each time someone said to me "oh I hope X didn't feel uncomfortable", even though they'd been telling stories of their own school days.
I think for most these days, that's a huge part of the reason for mentally sorting people into one box or another. Not to exclude or be stand offish if someone is from a different background, but to be sure they don't put their foot in it and make someone uncomfortable.
I'm not expressing myself well here, but does it make any sense?