The Elfin Knight, Child #2, in a version sung by Bridget Geary of Waterford, Ireland, 1906, called "The Sea Side."
"The Knight and the Chief's daughter," "learned from an Irish nurse." (#4, Lady Isabel and the Elf Knight).

A number of versions sung in Ireland can be found by thumbing through Bronson, "The Singing Tradition of Child's Popular ballads."
If your key word is "widely sung," however, I dunno.