In fact, it is on the DT, here. Possibly Yeats published two slightly differing versions; that in the Oxford Book of English Verse has "Salley" rather than "Sally", and, in verse 1, "pass'd" instead of "crossed" and "But I, being young and foolish, with her did not agree". In verse two line 1, "down" is omitted. Yeats said at the time that his poem was based on an older song (not a question of plagiarism it was common practice to write verse based on older sources, not least so among Irish poets of his generation) and that is generally held to be You Rambling Boys of Pleasure. A couple of earlier threads have some interesting discussion on the subject; Sally Garden and Song history/origin: Sally Gardens