I, too, have heard (and seen!) Colum do this several times. I remember the tune as a variant of "The man who waters the workers' beer", but my memory for tunes isn't too reliable.

Slight amendment of chorus:
A mule it has two feet behind
before(?) two feet before
don't stand behind before you find
what the feet behind be for

I work with close friend of Colum's who may remember more. I'll try to ask him before he goes off to Rostrevor in two weeks' time.