You might find it through one of these sites: http://www.smo.uhi.ac.uk/gaeilge/gaeilge.html#seanseanGhaeilge
Corpus of Electronic Texts
Kuno Meyer, Selections from Ancient Irish Poetry (London 1911), books of old Irish poetry by Gerald Murphy and by James Carney. I can't promise Heavenly Banquet is in any of these, but they are likely sources.