Sevenwaters, book 3: Child of the prophecy
Juliet Marillier2002

The daughter of a forbidden romance, Fianne has been raised in isolation and trained in magic by her loving but remote druid-father, Ciaran, and her ruthless sorceress-grandmother, the Lady Oonagh. They send Fianne to Sweetwaters to live among relatives who had no knowledge of her existence and who may have instigated the death of her mother, their sister Niamh. Fianne has come to carry out her grandmother's long-planned vengeance on the clan--and on the Old Ones, who are the source of Ireland's mystic power. Despite her mother's death, Fianne is reluctant to harm her Sweetwaters kin. But if she lets them live, the Lady Oonagh will kill both her father and Darragh, the handsome young horse tamer who has captured Fianne's heart.