"Tom Lenihan is correct about the origin of the rose-briar motif."
You have to be careful about ascribing 'origins' to anything in folk-song (that's what has caused a number of problems here). In the end it boils down to what is 'right' for the singer; where he first heard it, or what his source had told him about it, for example.
An incident that took place when we had recorded a song from him once put it in perspective for us.
When he finished singing he said, "That's a true song".
We asked him when and where he thought the story of the song had taken place; he looked puzzled and said "do you think it really happened?"
His concept of "true" wasn't quite what we thought he meant.
Jim Carroll