It was originally a harp piece "Plantxy Anacreon" by Carolan, to which the famous Irish poet Ruarigh O'Prutcoimh set these words:

ALL FOR ME POITIN

Where is me shilelagh,
Me naoichian*, naoichian shilelagh...

* meaning roughly "misplaced during a brawl"

And I can give you a good price on a fragment of the Blarney Stone.