I usually sing the whole song in Gaelic, and finish with a reprise of the first stanza in English for those not in the know.

The lyrics cited by AlastairG on 11 Dec 2012 are the Kenneth MacLeod translation from "Songs of the Hebrides". I can't find the Jean Redpath recording on the net right now, but the alternative English title given for Tha mi sgith on her album "Laddie Lie Near Me" is "A Fairy Love Song", which is how it appears in "Songs of Hebrides", so I assume that's the translation she used.