Don't know why above message got posted twice!I won't argue scottish songs with anyone named Sandy, but my source is an Alex Beaton CD containing the song, which includes the lyrics and annotations of the unusual words. It defines "lay" as I have above and spells pibrock, which it defines as a slow,sad song or dirge(and so does my dictionary!)
Since a lot of "real" scots have no use for Alex Beaton, maybe he is not that good an authority. He plays and sings good, though!
Tom