Jim Bainbridge, in this case it's easy to define the original version. Tannahill printed his lyrics before he died in 1810. It was reprinted a few times after that but his friend R A Smith set it to the tune recorded by Elma Gluck and recorded many times later ... it is the tune in Johnson's Musical Museum and in Norman Buchan's wee red book so the original version is there for all to see. When I was researching Tannahill's poem/song about 10 years ago I looked at Elizabeth Cronin's version in a book in the British library but it was a hybrid version .... many appeared in the early 19th century onwards as Tannahill's song was conflated with Northern Irishman Hugh McWilliam's "Lass among the heather". many versions were collected in the North East of Scotland under the title of "The fair of Ballymena", a mixter maxter of Tannahill, McWilliams and Trad. Some well know Traditional singers sometimes called their version the Braes o Balquhidder hence so much confusion.