Considering how far this local piece has travelled around the world, it doesn't vary that much in oral tradition, suggesting that the broadsides from the early 19th century are the origins. On balance I would say it is probably all fiction, but some of the details give it the ring of truth.
Off the top of my head I think one of the earliest broadsides implies that the girl was originally from Portland which would make some sense as coastal communities were always closely linked particularly east and south coast. On the other hand it may just have been a broadside writer's ploy to increase sales on the south coast.
I spent many hours in the North Yorkshire Record Office at Northallerton
trying to fit the information in the ballad to deaths and burials at Bay
in the early 19th century but nothing matched.