There is another twist in the tale regarding Edward Bunting's collection

This page on The Irish Music Manuscripts of Edward Bunting: An Introduction and Catalogue includes the following comment:

"However, in his published arrangements Bunting altered the original melodies to suit the tastes of his public. He published only about a quarter of the music and music information he collected or was given, and seemingly for political reasons he published none of the traditional song texts, most of them in the Irish language, which he collected through agents such as Patrick Lynch of Co Down."

So there may have been original lyrics but Bunting did not publish them, and he collected the tunes by listening to them being played at harp performances and creating his own music notation so he may have perceived the tune to be in 6/8 time or preferred it in 6/8 time.