Concerning the history of internecine violence in Northern Ireland:

I've heard that one of the difficulties is the great number of varied accents. If you're from somewhere else, your ordinary conversation rubs that fact into others' faces, just by your accent.

What degree of truth is there in this?

I've not heard this but it sounds unlikely. The problems can be between people on the opposite sides of the same street and I wouldn't have thought accent had much to do with it.

"An Englishman's way of speaking absolutely classifies him ..."

Alan Jay Lerner quotes

It's certainly true that there are many different local accents in Ireland, just as there are in England, Wales, and Scotland. In the same way that most accents in Britain appear 'English' to, say, an American, most English people just hear an 'Irish' accent, although many can at least tell whether the speaker comes from the North or South.

In addition, there are certain shibboleths that enable the hearer to tell if the speaker comes from a Catholic or Protestant background. For example, the letter 'H' is pronounced 'aitch' by Protestants, 'haitch' by Catholics.