John, is that line in the original? I've heard "fought with Cathal Brugha", which is more ambiguous, and "fought with Valera true", which does even less to fix the time of writing after 1922.

Your quotation from the Down & Connor Diocesan Archives above gives no date of authorship. Also, contrary to Martin Ryan's post of 12 Nov. 1997, Cathal O'Boyle doesn't claim the song was written in 1919:

"The words of this song were composed by Canon Charles O Neill, who was parish priest of Kilcoo and later of Newcastle. In 1919 he went to Dublin and attended a sitting of the first Dail Eireann (Irish Parliament). He was moved by the number of members whose names were answered during roll call by "faoi ghlas ag na Gaill" (locked up by the foreigners) and resolved to write a song in commemoration of the Easter Rebellion. I have seen his song printed many times but have never seen his name mentioned and I think it is about time he was recognised. The music belongs to an old love song, recorded in 1913 by John McCormack and the original manuscript of the words and music, in the possession of Kathleen Dallat of Ballycastle, names Carl Hardebeck as the arranger." (Cathal O'Boyle, 'Songs of the County Down')