It's called "The Song of Wandering AENGUS", written by Yeats in 1897 and first published in his collection "The Wind Among the Reeds" (1899). The first line is actually "I went OUT to the hazel wood" not "DOWN". The collection also includes "The Host of the Air", first line, "O'Driscoll drove with a song, the wild duck and the drake" which was made famous by the Clancy Bros and Tommy Makem in the 60s. On the Grapevine label CD "Now and in Time to Be - a musical celebration of the works of W.B.Yeats" (1997), the music is credited as follows, "Music by Christy Moore (Bal Music)". I don't know if the album is available in "foreign parts". If anyone can't get the tracks let me know and I'll post them. All the best.