My former (and sometime) singing partner Mary Mc Laughlin sings a beautiful version of the lyrics posted by Felipa, and has made quite a study of the story, too. We have teamed it up with my song about a Welsh fisherman whose song attracts a Welsh mermaid for occasional performances. IIRC, Mary's version of the story is that Mary Chinidh is a mermaid who has come to shore for love of her husband and has children with him. One child falls into the water, and Mary jumps in to save the child but in the process is doomed to revert again to being a mermaid and never see her children and husband again.