Malcolm thank you for all those links. This song seems to have led a very confusing existence, leading to some wildly varying interpretations. I had meant to credit The Folksongs of Britain and Ireland as the source of the text, but was too tired to do so at the time, so thanks again. The translation given in FSBI is a bit misleading but the one I have given is based on it with the help of an Irish dictionary and some dim memories from lessons at school 30 years ago! I would be most grateful for someone who can improve it.
The droit de seigneur interpretation seems to me to be a bit off the mark, as most Irish versions of the song seem to refer to the seducer as a gruagach (trans. = enchanter). This seems to suggest that the young woman in the song is being put under a sort of spell, and being carried off for nine months(!) to the spirit world. I'm not too sure about the Persephone motif, but as Martin has noted the song is often sung to the refrain Thugamar féin an samradh linn(We have brought the summer with us), which celebrates the Bábóg na Bealtaine, or May doll.

I just posted this song as a penance for peddling crude jokes and shaggy dog tales on the cat-naming thread. It looks as if this could get interesting.