Many of us have researched this song thoroughly and none to my knowledge have found any reliable link to the song prior to the mid 60's when Behan recorded it. I think it is fair therefore to say it's not a traditional song. The various mysteries about the words I think can be explained. Behan said he wrote the middle verse and the lack of mystery in that verse alone I think proves it. Also he says he weote it down dictated by Peter O'Toole. Now I think we all know of his and Behan's liking of a drink,so you have two probable merry Irish men one trying to sing the words to the other trying to write them down, a situation not designed for much accuracy!
Add to that O'Tole was not a song writer so it seems this was a song he had heard somewhere and aren't songs transferred by one to another in that way part of the folk tradition whereby words and meanings rarely stay as they were meant to. 'The water is wide' is one of the most copied lines in Irish music and appears in some form in several Irish songs, the tune too is very likely traditional. The only real mystety for me is, Where did O'Toole find the song?