G'day,
I heard a version in Welsh a while ago with a translation of the story as a preamble. I don't remember the details but apparently the person who "sleeps 'neath the green turf" had been murdered. Then I seem to remember that it was a young man who was dead (in spite of the translation that goes "she sleeps...."), killed by the girl's father/brothers for the traditional reasons.

I first learnt the song in 1956 (when I was very young of course) and it's had an effect on me ever since, it always seemed very mysterious.

Cheers,
Alan