thesession.org has versions of Planxty Irwin in both G and D (and, for some reason, C). I believe I learned it from O'Neil's, where it's in D (I think. My copy of O'Neil's is buried somewhere and we're painting the two front rooms and everything's piled in a heap.)
One of the contributers said they play it in G if whistle players show up, but would rather play in D when there are fiddle and mandolin players.