"Adapted from" still leaves it open as to whether that means that Padraic Colum passed on the words with minimal changes, or whether he used the older ballad as something out of which to make a new song.

After all every sculptor carving a statue is adapting it from a bit of rock. Cutting out the stuff you don't need is an important part of the creative process. As with Yeats' reworking of the song he trimmed down into "Down by the Sally Gardens", or the transformation by Francis McPeake of The Braes of Balquidder into "Will ye go lassie go". Or the way that selective memory and so forth over the years has transformed any number of songs from doggerel into poetry.

Salix is the Latin for willow, and "saileach" is the Irish, so that seems to bear out Happiness's idea. And it gives an additional resonance to the song, since willow trees are supposed to be particularly beloved by water spirits. (And with Yeats being so into magic and the folklore of it, that might well have been what he was thinking about in the rest of it, what with the grass growing on the weirs and all.)