I’m thinking that the “rogue” refers to the singer.
“A child of Mary from the rebel liberties” is Peggy. She’s a good Catholic, but her family background is that her parents or grandparents likely supported the DeValera side in the Civil War of 1922.
Sounds to me that she more than dated “a black guy”, she went off with him to Birmingham, in England. Ah, but was he Negroid?
The area around Brimingham is known as “the Black country” for the coal mining that goes on there. The individual in question was a student, we are told, but he also had “a skin as black as coal”, which to me would mean that he may not be destined to be a coal miner himself, but that coal miners exist in his family or background.