It fascinates me too, Amos. Malcolm has said it better than I could but I reckon it's the mark of a great poem (or song) that the simplicity of its expression allows wide variation in interpretation, which would mean it could be applied to a wide range of situations.

Early in Melbourne's folk scene revival it was routinely sang as the last song of the evening. While occasionally someone dropped off the perch most of us survived until the next gathering and I never heard anyone associate it with death/hanging/military service/ghostly presences departing or any of the other fates mentioned above.

Just a more melodic and expressive communal way of saying "Hooroo."

Cheers, Rowan