The singer is leaving Donegal and Mary from Dungloe for America. "It was your cruel father" that drove him from her but he plans to return. "I wished I was in sweet Dungloe and seated on the grass And by my side a bottle of wine and on my knee a lass"
The Digital Tradition lists this to the tune of "The Star of the County Down." The version I've heard (admittedly from The Irish Rovers) isn't quite that, but it may be a variant. - RBW