About the tune: I looked up some info about James Hogg. He was a poet only, and didn't write any tunes. So apparently he wrote this poem and some time later somebody saw it and said, "That would make a pretty song. I'll make up a tune for it." And they did.
This could have happened many, many years after the poem was written.
Anybody else can do the same, including you or me. So don't wait for somebody to give you 'the' tune because you might never get it. Make up a tune, sing the song, and bring a little beauty into your t world.