The nonsense in the chorus' of irish and Scottish songs seems to be just that - nonsense. You can see the same thing in Puirt a Beul - "i/ bhi/ a bhi/ u/ bhi/ a bhi/" (ee vi ar vi oo vi ar vi) though bhi/ is a word, or in Domhnaill Antaidh "hi/ ri/ iu/l eile".
Often they are just sounds that allow the rhythym to be kept and that large numbers of people can sing. That's my thought anyway. I actually tried to write the words for Whiskey's chorus in Irish and then translate and I never had much success.