Brian,
Just sounding a warning about tunes - too many definitive statements being made here already without my adding to them.
I suggest you get hold of some of the John Reilly recordings to hear a singer not only changing the tunes, but also the verse structure.
One of the finest examples of mode change was the late Tom Costello (Tom Paidín Tom) singing (breathtakingly beautifully) The Grand Conversation on Napoleon.
Jim Carroll