It's now very easy to find the main thread about this song; see "related threads" at the top of the page.
If you read the other thread you will see that the original Irish language song is attributed to John McCambridge (Seán Mac Ambrois)and the best-known and recorded translation to Joan O'Hara (sister of the well-known singer Mary O'Hara). As suggested, that translation may well be under copyright (even though it seems some of the recordings don't attribute the translator). I haven't heard the Fureys sing this song.