The majority of examples found in tradition seem to have come from non-Gaelic speakers who had no idea what, if anything, those words might mean, and "mangled" forms of the refrain are pretty much the norm rather than the exception in the case of this particular song. In many cases they have undergone considerable change in the course of transmission, and are de facto no less "authentic" than other (rarer) examples which have retained the words in a coherent form. In the case of many variants, attempting to revert to an apparently earlier form or to use "correct" pronounciation would, arguably, be quite inappropriate.

That wouldn't necessarily be true of a lot of songs, but this one is perhaps something of an exception; probably no point in worrying about pronounciation, as it comes, so to speak, "pre-mangled".