> To continue the thread drift, I'm enchanted by Marta Sebestyen singing Leaving Derry Quay

An interesting example! Here we have someone whose native language is not any variety of English singing an Irish song. From the singing alone she could almost pass for Irish, having only slight differences in the accent. All well and good. The only thing wrong with it is that the accompaniment is a tad too loud relative to the voice, making the words a bit harder to understand.

Now, what if she had sung in RP or some sort of American English instead? That would seem to make no sense.

Or what if she (or any non-native English speaker) had lived somewhere in the English-speaknig world and become fluent in the local variety of English, e.g. Liverpool, New England, Kentucky ... and then sung an Irish song? I myself am unsure of the right answer to that one.