Thank you so much for all the info! I love the lyrics to "maa bonny lad", so simple and sweet and with a hint of sadness, just perfect for the tune. I guess Olle Adolphsson must have heard someone sing "maa bonny lad" in northern dialect, to untrained Swedish ears it would sound like Scottish. One funny thing: the same friend who insists the tune must have come from "Where the three counties meet" actually has "Billy from Sligo" in one of her Irish session books but when we played together that's one of the tunes she would always skip because she thought it was boring - so there she had an answer for my quest for many years and never realised it!
Again with many thanks and if you find more info I'll be even more grateful :)
/Anna Malena