I wouldn't for a minute that Christy was too big a claim for himself. I assume that he hadn't heard it sung anywhere and came across it some book or other. (Is it, for example, in Paddy Galvin's Irish Songs of Resistance?) And I've no doubt that Christy's singing of the song popularised it immensely. There's many a song that nobody in Ireland would ever have heard of had it not been for Christy Moore.