The "Irish Rovers" simply recorded an arrangement of Glyn Hughes' song. It is neither traditional nor Irish, and the books Q mentions don't relate to it.

There is a pretty good search engine here, and it's a pity that so few people bother to use it. See, for example:

WHISKEY ON A SUNDAY or COME DAY, GO DAY (Glyn Hughes) (DT file)
Chord Req: Whiskey On A Sunday
Whisky on a Sunday
Who Wrote The Ballad of Seth Davy
Seth Davy info please
Seth Davey
Who is/was Glyn Hughes

Whether any provide accurate lyrics, I don't know. Many certainly contain misinformation. Q's link to that "Liverpool Lyrics" site may be as good as you'll get, though with various mis-spellings.