Anyone wanting authoritative guidance on how to play any Carolan tune could do worse than consult Caitriona (sorry, I can't remember how to do the fada) Rowsome's "The Complete Carolan Songs & Airs Arranged for the Irish Harp". FWIW she calls it "Fanny Power" (without the "Planxty") and notates it as 6/8. She gives a bass line (rather than chords, these being harp settings) having two groups of three beats, not three twos.