Scabby Doug raises a very interesting point here. We are not told the exact Latin that confirms the "Pictish" attribution, but assumingit was used it is certainly true that the word "Pictish" was used very loosely in those days. Quite possibly the collector who made these notes was not that knowledgable on ethnomusicology and used the term erroneously in referring to Macdonaldus. Of more interest is Scabby Doug's point that there shouldn't be anybody called Macdonaldus near the Roman Wall at the time. I would suggest the possibility of seasonal migratory agricultural workers, who came over regularly from Donegal and Antrim.
However, there is a much more intriguing possibility which we have to consider seriously.It is often assumed erroneously that the Romans never got to Ireland. In fact, thoughthey never made any serious colonisation attempts, there were many exporatory trips, and recent archaeology has in fact uncovered temporary Roman forts thrown up during those forays. So perhaps the song collector had in fact been on one of these expeditions and picked the song up in Ireland, in which case we may have here the earliest known Irish song, albeit in Latin translation.