I think you're right about these lyrics being just an interpretation by a (possibly non-Oz) fan. The reference to old Brad is meant to be Old "Red" (Ned's dad). And to get really picky, the last line in the chorus should read "Boy, they're gonna hang you high".
Despite the inaccuracies, I relly do prefer the Fotheringay version.
So, you're a Quinn, huh? Remember the days when it was considered "not done" to reveal your convict ancestry. I gather that the descendants of the families of the Kelly clan around the Greta area still don't talk much.