Given the citation from the Clancy songbook, I'd hazard a guess that they would readily admit it's a traditional tune that they adapted and arranged.

With that in mind, what is said above is correct - if it's traditional, all you need to be able to prove is that the song existed before the copyrighted arrangement and that your version is based from tradition instead of being a copy of the newer version. Of course, unless you're doing a carbon copy version of someone else's arrangement, hopefully they would realize the folly of trying to chase you down for it (moreso on something very easily identified as traditional).

All that the "arranged/adapted" really signifies is that the group in question tweaked the tune and lyrics. The extent of the tweaking isn't all that important, but by doing so they have the right to copyright that arrangement, but not take credit for having written it as an original song.

M