I'd be surprised if Behan hadn't known the tune was traditional: he fequently used or adapted well-known tunes, like so many other songwriters. But I expect he was pretty pissed off that Dylan's words was a spoof or parody of his. Alternatively, he could have decided imitation was the sincerest form of flattery, but I doubt it somehow!

Many of Dylan's songs took a trad song as a starting point--nothing wrong with that, in principle; but when does "borrowing" become "ripping off"? Dylan said he didn't just change songs for the sake of it, but he didn't actually explain why he did it. But then it never bothered me much at the time!

Steve