Well, for what it's worth, here's what Kuno Meyer wrote in the notes in his book Selections from Ancient Irish Poetry Translated, published 1911:

For the text and translation see Stokes and Strachan, Thesaurus Paleohibernicus (University Press, Cambridge), vol ii, p354. I have adapted the translation there given in some details. The hymn in the form in which it has come down to us cannot be earlier than the eighth century.

Mind, I'm sure you could find it in a more recent book than that. It's a major text of Irish literature. Enter The Deer's Cry in the right search engines, and you should find all kinds of stuff.

As for the tune, I'm inclined to doubt you'd find any tune that can be authenticated as being used with this in the eighth century, let alone a few hundred years earlier when St Patrick was on the go. But authenticity isn't the central thing anyway, fittingness is.