Masato,

The Dubliners claim they got it from Joe Heaney who claims it to be related to a very long song in Irish that he sings a few verses of on the Ossian LP 'Songs in English and Irish' (I think). As I remember the title is 'Peigín agus Peadar' and is about a farmer going away to work 21 years for a richer farmer and coming home to find a bearded fella in bed with his wife - ostensibly his grown-up son. Same premise as 7 drunken nights minus the drink and with a bit more ambiguity thrown in. I'm sure it's a fairly generic theme anyway.