Niamh was the fairy woman from the Land of Youth who lured Oisín, the son of Fionn, away from Ireland on a white horse. After a period of intense happiness with her there he got homesick for Ireland and Niamh allowed him borrow the white horse for a return journey. She warned him not to touch the soil of Ireland if he wished to return to her. The story of Oisín's return and the tragedy that befell him is to be found here http://www.sacred-texts.com/neu/celt/gafm/gafm87.htm in a version by Lady Gregory.