The music to Mary Mack appears in a book I have called
"100 Great Scottish Songs with words and music"
Published by Soodlum, the book comes complete with a cassette containing at least the first verse and chorus of every song, sung by Sheila McWhirter, Andy M. Stewart, Cilla Fisher and Davey Steel.
The contact addresses given are:
Walton's Manufacturing Ltd. River Valley House Kylemore Park South Ballyfermot DUBLIN 10 Ireland
or
Walton Music Inc. PO Box 1505 Westfield MA 01086 - 1505 USA
They also publish 100 Irish Ballads with words, music and guitar chords, Volumes 1 and 2, both of which are great resources too.