Dick, you're correct of course, but as I've pointed out many times before, Dylan parodied Behan's words to the same tune, proving he, Dylan, didn't hear the tune from the traditional Nightingale song but from Dominic's Patriot Game. I Know this as a fact as Dylan asked me about the Patriot Game when he heard Nigel Denver singing it in the Troubadour many years ago.
I think we should consider Dylan as a re-writer rather than a writer.