The Sword of Light by Heather Graham

Will fabled objects of myth be enough to defend their emerald isle? The mystical Tuatha Dé Danann walked the hills of ancient Ireland long before the first Rí ever ruled the land. … Deidre, the daughter of a Rí, and Kylin, the son of a Northman who found peace and a home in Éire, have little in common until they begin seeing visions of these fantastical gifts. The revelation that a traitor exists among the many Irish kings forces them to work together using their newfound powers in defense of their homeland.