> Padraic Colum didn't write the poem,

In fact he did. All but the last stanza. The poem appears as his own work on p. 26 of Colum's "Wild Earth and Other Poems" (1916).

Colum and the musician Herbert Hughes combined the poem with a melody that Hughes had collected in County Donegal. Colum's lyrics "adapted from an old ballad" first appeared in Hughes's "Irish Country Songs" (1909), pp. 46-48.

Colum wrote in 1970 that only the final stanza had been from tradition.