I was listening to the Irish folk song Gilgarry Mountain (aka Kilgary Mountain, aka Darlin' Sportin' Jenny, aka Whiskey in the Jar).  The chorus goes:

Musha rig um du rum da
Whack fol the daddy o
Whack fol the daddy o
There's whiskey in the jar

Anyone here know what this means?  Google doesn't help much; I've found only two references to a possible meaning.  One person though it was some kind of nautical slang, but I'm fairly well versed in naval terminology, but I've never heard of anything remotely similar.  Another person thought it sounded like drunken Gaelic for "the work of the Devil".