The tune associated with Yeats's 1889 poem used to be known as "The Maids of Mourne Shore," published by George Petrie in 1855. It would need some adjustment to be fitted to the "Rambling Boys" words. Am not sure who assigned the tune to Yeats's words.

Yeats's rewrite of a traditional song based on the imperfect recollection of a few lines is a perfect example of the "folk process." So is the application of a tune. The stellar result is a bonus.