I spoke to Robbie McMahon at the weekend, during a festival in Feakle, Co. Clare. His recollection is that he was told as a child that Michael Considine's poem had been set to its tune, locally, in Spancil Hill. He has never heard the tune used for anything else - nor does he recall any mention of what it had been previously used for. "It was just lying around, really!" was his comment.
Sounds like the trail is cold, I'm afraid.
So, we have three sets of words and one tune (sung two ways).