"Roughly when did he learn them?"
I think it would have have been around the beginning of the last century - I understand he was one of the founder members of the seaman's branch of The Workers Educational Association.
Interestingly, he and my grandmother were fanatical anti-racists (my grandmother was arrested for throwing a brick at Mosely), yet he had no compunction singing "n*****" - not a term I've ever been commfortable with.
He also sang 'Johnny come down from Hilo' which started:

"I've never seen the like since I was born,
As a big buck n***** with his sea-boots on."

The Bulgine was another one of his.
I wasn't interested enough at the time to memorise his songs, but unknown to me, my father's brother, my Uncle Gerry remembered most of them and sang them at the whitby Folk Festival - I didn't find this out until after he died.
I've alwys dreamed of finding that somebody recorded them from him - and regretted I didn't.
Jim Carroll