It's interesting in NI because haitch-vs-aitch is so much more obvious than any of the other common shibboleths that it's become almost a comically exaggerated

It's interesting in NI because haitch-vs-aitch is so much more obvious than any of the other common shibboleths that it's become almost a comically exaggerated thing - something that people feel free to comment on or even joke about in a way that they wouldn't about anything else. (Derry vs Londonderry might beat it for sheer obviousness, except for the fact that no-one, even the most hardened Unionist, actually calls it Londonderry in everyday speech unless they're making a point... or work for the BBC) But, in reality, I think it originally comes from Hiberno-English vs Mid Ulster English, the two main dialects of English spoken in Ireland, rather than anything to do with religion. The association with religion came with the post-famine wave of emigration to Britain, and it was only after that that it began to filter back to Ireland. (That's the story I was told at school, anyway...)