A New Commentary on the Poems of W B Yeats by A Norman Jeffers (1968) says;

Yeats's footnote explained that 'This is an attempt to reconstruct an old song from three lines imperfectly remembered by an old peasant woman in the village of Ballysodare, Sligo.'

[H E Shields] argues that it is 'The Rambling Boys of Pleasure', an Anglo-Irish broadside ballad, the text of which is in the National Library, Dublin. Mr Shields gives several versions and says it was written to the Irish metre of, for example, An beinnsin luachrua; he obtained his source from Mr Paddy Tunney.