I've done this a few times (which will not go over well with some folks), setting my own words (in English) to the airs (melodies) of Irish-language trad songs. Traditional music is not subject to copyright laws while arrangements of such may be.

Probably the best way to find out whether an air is traditional is to start a thread here, naming the air, and our knowledgeable scholars will tell you.

While anonymous Guest postings are permitted at Mudcat, you will garner much more good will by using a consistent "handle".