Sandra asked if I could check out the Eric Bogle Songbooks for the tune ... I did ... No luck! I don't think he has put out a songbook for a decade ... or more ... and this song is not in any of the three I have.
I do have a friend who sings this (when he is not being employed as a "Plastic Paddy"!) but I think he just learned it by osmosis - and he is too busy trying to earn a crust as a singer and a luthier to spend much time trying to convert it to dots and scratches. I will see if he has some sort of tune/chords ... when we cross paths next.