Further to Dave's post, I have also come to the conclusion that Francis McPeake obviously adapted Tannahill's lyrics but his tune is either traditional but noted nowhere else, or that he composed it himself. He had a great deal of old tunes in his head as a traditional musician so he could easily have composed it as an original work.
I knew The McPeakes personally and have always considered the song as an original by them with lyrics based on Tannahill's.