As Bruce Olson posted long ago about this (quoting an email from me) Tannahill did no more than slightly adapt a song by John Hamilton from 1792. Its tune was published by Bremner around 1760, which probably means the words, in some form, go back that far too.

I think McPeake's sole contribution is the "I will surely find another" bit, which is crap anyway and the song is better without it.