I don't think anyone here has quoted at length what Christy Moore says about the song in his book, One Voice – My Life In Song:

Many have claimed both authorship and discovery of this beautiful Scottish ballad. Truth is it was probably written by Robbie Burns. In 1962 I discovered it in the Joyce collection. I brought it round to Donal Lunny and we worked in a chorus gleaned from the third and fourth line of verse three. If memory serves me, I gave the song to Mick Moloney .... He subsequently recorded with The Johnstons and that wonderful version established the song in the national repertoire.

.... I gleaned a number of songs from [P. W. Joyce's collection of Irish music and songs] – among them this jewel. This was back in 1962 and Donal Lunny and I worked it up into what it has become. Various ballad and folk bands have subsequently taken it to a million ears – one noted Irish singer even claimed to have written it, but Rabbie will ne'er be denied. Burns 'twas and Burns 'twill be.

P.S. I've just discovered that there is some doubt about Robbie Burns' authorship here. I can neither confirm nor deny at this time. C.M. 10/11/99