Thanks, Martin. That story has been mentioned briefly in another thread, also the supposed tie-in to Patrck Donnelly.
And thanks to Fiolar for checking lyrics on McCormack.

There also is little evidence that the song was ever widespread (not in collections of Sam Henry, O Lochlainn, Graves.

Seems like one of those retrospective 19th c. songs published on broadsides, and taken up in the 20th c. by folk singers who search through broadsides.

The tune well could be older, and, like so many, fitted or revised for a later lyric.

McCormack obtained his version of "Bard of Armagh" and other Irish songs from Herbert Hughes (graduated Royal College of Music in 1901), a folk song collector and composer who published several volumes of Irish songs and arranged them for concert performance. His arrangement of "Down by the Sally Gardens" is perhaps his best-known.