"Húg" seems to remain a mistery then, unless it's a misspelling a "thúg", but it's unlikely. I have the original CD with the song and that's how the title is spelled. I know that "h" can be used sometimes at the beginning of a word in some cases, when the word starts with a vowel, as in "Óglaigh na hÉireann". But this doesn't seem to be the case here either, does it?
"Húg" is not a word in Irish (to the best of my knowledge) but "thúg" sounds like "húg". That's why I assumed that it was a misspelling, but perhaps it's something to do with the difference between Irish and Scots Gaelic.