The Dark Mirror (Bridei Trilogy)

Bridei is a young nobleman, a fosterling of one of the most powerful druids in the land, Broichan. All of Bridei's earliest memories are of this dark and mysterious man he trusts above all others. Broichan seems to have plans for Bridei, seems to be training him for a special purpose the older man will not divulge.
One bitter Midwinter's Eve, everything changes when Bridei finds a child on their doorstep - a child abandoned by the Fair Folk.
It is the height of ill fortune to have truck with the Fair Folk, and all in the area counsel the babe's death. But Bridei sees an old and precious magic at work and, heedless of the danger, fights to save the child. Broichan is wary but relents, for Bridei must grow to be his own man and make his own decisions.
As Bridei comes to manhood, he watches the shy girl Tuala blossom into a beautiful woman, and feels things he didn't know were possible.
Broichan sees the same process and feels only danger, for Tuala could be a key part in Bridei's futureor could spell his doom.