I’m afraid I cannot accept any similarity between McPeakes’ tune and the version by Alma Gluck which is note for note of that printed in any Victorian song book, copied from R A Smith’s publication. No amount of wishful thinking and guess work will achieve anything else.
The McPeakes obviously used Tannahill’s lyrics but that was all.
The McPeakes produced a song, original in melodic content which belongs to them completely, no matter whether an uncle or the next man’s dog wrote it……they aired it first and no one has proved otherwise.