That would be because, so far as can be told, there aren't any Gaelic lyrics. As mentioned above, the tune had a Latin title for the first couple of hundred years of its life; the Gaelic and English titles first appear at about the same time. There seems to be no record of Rory Dall having made words for his composition (if he had, he'd have been as likely to use English -or even Latin- as Gaelic), and songs set to it appear all to be modern; of course someone may well have written a Gaelic lyric for it at some point, and perhaps someone else will know of one.