im a kiwi of irish descent, my grandmother was from galway and a gaelic speaker, my grandfather was born here but had come from a long line of only irish, 7 generations in fact, im 9th.
he didnt really speak gaelic but he was a staunch republican and liked a drink with the lads, he said to me that 'musha rig darum do damada' is gaelic for 'oh well, i fucked up' and that 'whack fol de daddio' is 'drink to your fathers/ancestors' referring to those that died fighting the english, as the hero of the song is resigning himself to doing, now that he is an outlaw.
he actually asked my grandmother for clarification on that, she concurred saying its nonsense words but must have meant something like that originally, before getting mashed by non-gaelic speakers...