The Witches of Eileanan, book 1Kate Forsyth1997

In the Celtic land of Eileanan, witches and magic have been outlawed, and those caught for practicing witchcraft are put to death! It is a land ruled by an evil Queen, where sea-dwelling Fairgean stir, and children vanish in the night. But in a valley deep in the mountains, young Isabeau grows to womanhood under the guidance of an elderly witch, and must set out on a quest, carrying the last hopes of the persecuted witches.