It's nice to know who wrote this song and who changed that song etc, but lets not forget what is important. Folk Songs of love, history, murder, mayhem, rebellion etc, are being sung, by me, by you, and countless others, thereby preserving these songs for future generations to enjoy.I don't really care if McPeake wrote it, stole it, changed it - changing words is part of folk music and hopefully will continue. This is a great song and will be around for many years to come.
Lets be thankful for Tannahill, McPeake and others like them who continued to write, collect, sing and play traditional songs and tunes even when it was not the in thing to do. Look at the treasure they left all of us.