I think I might be able to help solve this one.
A singer at our local club often does the song. We went over to Ireland last year and on the way back to the ferry passing through co Waterford we saw Dungarvan signposted so we thought we've got a bit of time left, let's go and investigate.
A lovely place and we really enjoyed going around the museum, but could find no info at all on the oak tree.
The plot thickened a bit later when we met a friend of ours Taff Brissenden at Shrewsbury in a singaround he had organised. He did a song called "The Old Carmarthen Oak" which was exactly the some except for the substitution of name of the town.
It turned out that Frank Hennesey wrote the song as "The Old Carmarthen Oak" - I think the confusion might have arisen as he has an Irish sounding name but in fact the song was written about Carmarthen.
It would seem that Daniel O' Donnell is the villain in all this for blatant plagiarism as I believe he credits himself with the writing of the song.
Is it just a case of these Celts shamelessly stealing stuff from each other?
If you are visiting Dungarvan I can recommend taking a detour of the main road but you won't find the celebrated oak.