I haven't researched the song any further since - sorry. Thomas Campbell (1733-95) is certainly the right era, and area - well Tyrone, but, like Martin, I think it's unlikely that he wrote the song: there's no mention of him writing any songs in the Oxford Companion. Mid 19th C. was as far back as I could take it. It is tho generally accepted locally in South Armagh that the song is definitely about Bishop Donnelly.