Whether or not "Rosin" (as distinct from Roisin, Little Rose) was ever someone's name, the phrase "rosin the bow" makes perfect sense (and despite what someone said way up thread, what a string player uses is rosin, not resin). I'm inclined to regard "Rosin the beau" as a pun. I can imagine a fiddler who is a bit of a character being heard to mutter "I need to rosin the bow" so often that "rosin the bow/beau" became his nickname.

Not that this tells us anything about when, where or by whom the song was first made.