On the Edge of Darkness by Barbara Erskine

A COMPELLING STORY OF A WOMAN TRAPPED IN THE WRONG TIME, AND A POWERFUL, DEADLY CURSE THAT HAUNTS THREE GENERATIONS WITHOUT MERCY...
Adam Craig is fourteen when, near an isolated Celtic Stone in the wild Scottish Highlands, he meets Brid, whose exotic, gypsy-like dress and strange attitudes fascinate him. They become friends and, in time, passionate lovers. She leads him, unsuspecting, into the sixth century where - training as a Druid priestess - she has mastered their ancient mysteries and powerful, dangerous magic.
In her obsession with Adam, Brid is seen as a traitor to her people, only escaping death by following Adam to Edinburgh when he leaves home to study medicine.
As the years pass, Adam makes new friends, and finds new love. But Brid, consumed by jealousy, haunts him like an evil shadow until, fifty years on, Adam's granddaughter, Beth, helps him discover the secret that will free them from the terror of Brid's curse.