Gatting back to the "Irish" question.I believe, especially in the UK, that most of it can be traced back to the Dubliners. Their influence was almost all pervasive in the mid-sixties. Many people sang their songs, and a standard-folk club voice seemed to develop around that time as well. Which thankfully seems to have died off.

Regarding Scots/Irish, it's easier for a non-Irish person to sing an Irish song, because the words are mostly standard English, with an Irish accent. Some Scottish songs are the same, but the majority have Scottish words that are unfamiliar to the rest of the world, or more archaic pronounciations. It's much more difficult to make that sound authentic, but hey, good luck to those who are willing to try.