In Complainte pour Ste Catherine they use at least 3 and probably four part harmonies. (one above, and two below, the tune.)

In the recorded version of "Complainte pout Ste-Catherine," the harmonies are multi-tracked.

It struck me that one of those ladies (the McGarrigles) can sing awfully deep - for a lady that is.

Most of the McGarrigle recordings feature harmonies sung by males: Dane Lanken, Chaim Tannenbaum and Peter Weldon on the early sessions, also adding Rufus Wainwright and Joel Zifkin on some of the later ones.