My copy of O'Lochlainn gives two sources.(Unless I'm misinterpreting his shorthand).

1. "Piii, 693", [which is the Complete Petrie Collection]
2. "Pi, 134" [Petrie's Ancient Irish Music]. Petrie's works are dated between 1855 and 82.

O'Lochlainn says that the tune is a variant of "Dawning of the Day", so there indeed is a link to Patrick Kavanagh. (O'Lochlainn was a piper with a great ear for melodies)

O'Lochlainn's version is pretty standard, "Johnny went a thrashing the dirty King of Spain", etc.

Re the "Moneyhore" verse, this is a townland near Enniscorthy, Co. Wexford, renowned for its Fair, at which the Bantry girls were no longer going to enjoy Johnny's company. Fairs were often the scene of Faction Fighting, so the fact that "the Peelers must stand idle" perhaps suggests that Johnny was a renowned Faction Fighter.

In Aidan O'Hara's biography of her, Delia Murphy's recording of the song is dated 7 February 1941