Rather than respond to a message Martin Ryan sent me through Facebook, I think I'll answer here.

I don't think I can add anything except to clarify the existing record:

When Clannad issued their recording on 'Fuaime' they said: 'An old song in praise of the parish from which we come. Cathal Ó Baoill (Charlie Joe Thimlin) sang the words of this Song to us which we set to a County Derry air.'

They mentioned this Cathal Ó Baoill (who, from his family names, must be local) in their acknowledgements and mention also another Cathal Ó Baoill describing him as 'Muinteoir'. I don't know whether the latter refers to the Co Down teacher, song collector and son of Seán Ó Baoil but it is certain that any linking of the singer of this song with the song collector is not justified.

Martin also cited the Roud index:
GREEN FIELDS OF GWEEDORE, THE; Down by Dunlewy's bonny lake one morning I did stray        Sound recording
Ulster Folk & Transport Museum collection (Holywood, Co. Down) No.6943/6945 Performed by Heraghty, James; Co. Donegal : Cloghaneely        1969; collected by Meenan, John

I consulted Hugh and Lisa Shields 1975 Index which is the source of the Roud entry. It makes it clear that James Heraghty was recorded twice, firstly by John Meehan who came from Tory but was working in Meenalaragh, Cloghaneely in 1969, and then by Hugh Shields who also copied Meehan's recording and lodged both his own recording and the copy in UFTM.

I've checked all the books of local history and of local poetry that I have from this area and can find nothing. I can only suggest that you try to question the Brennans, Maireadh NÍ Mhaoinaigh, and anybody else from the area you might know. I might try to ring Hudie Beg Gallagher who is an old acquaintance.