I still don't think "Valentine" and "Newry Highwayman" are so closely related. Ned Fielding, as the magistrate who essentially established the police force, would naturally be mentioned by many highwaymen. But the context is different; in Valentine he gets the better of Fielding, in Newry Fielding's gang captures him. The songs have no real rhymes in common, etc. By most of the means by which we judge relationships, these songs are unrelated--except that, obviously, they are from the same era, the same country, and on the same popular theme.

Great thread!