This is Child Ballad #76 (The Lass of Roch Royal). It was first published by Herd in 1776. I've never seen any historical references but would be interested myself. There be doesn't seem to be any 'Roch Royal' but it's been suggested that it may relate to a Rough Castle near Falkirk and a Loch Ryan in south-west Scotland. Another common placename in other versions is 'Capoquin.'

Sorry - that's about it. If you learn any more please post back here. It's a very good song.