Book III of the Sevenwaters Trilogy.
From the cover:
"A rich, magical story of loyalty and love that recalls the ancient Celtic past to life.
Raised in an isolated cove on the beautiful Kerry coast, Fainne's childhood is a lonely one. But her beloved father teaches her all he knows of the magical arts, and every summer she looks forward to the arrival of her one friend, the gypsy boy, Darragh. Soon, though, her world will be changed forever by the arrival of her grandmother, Oonagh, the renowned and feared sorceress.
Oonagh tells Fainne that she carries the blood of a cursed line of sorcerers and outcasts, and then she burdens her with a terrible task. Fainne must go to Sevenwaters, to the family she has never known, and use her power to thwart a prophecy that is near fulfilment. If the child of the prophecy lives to reclaim the sacred isles for the Fair Folk, Fainne will feel the full force of Oonagh's wrathful desire for revenge.
Child of the Prophecy is a powerful and haunting conclusion to the Sevenwaters trilogy."