If it's no trad and fairly modern best to ask permission. BUt as I'm sure others have pointed out, there have been millions of tunes and logically they can't all be 'original' Many Irish tunes and country tunes are unwittingly copied and have standard patterns. Bob Dylan borrowed Dominic Behan's tune of The Patriot game to write God On Our Side. Behan did try to accuse him of Plageurism (spelling?) but Dy;an rightly ignored him as his lyrics are substantially different, and Besides Behan had copied an old trad tune in the first place.

But that is an important point, Dylan's Lyrics are sunstantially different and that to quote Pete Seeger is all part of the Folk process. If you use an existing tune and write a song on a similar theme you could be on legal dodgy ground. But if you're lyrics and/or theme then you're ok