Sidh Legend, book 1: The Riders of the Sidhe
Kenneth C. Flint

Out of the mists the Fomor came to enslave the isle of Eire, a dread race of twisted men ruled by an inhuman lord: Balor of the Evil Eye. But a champion came from out of the sea, a youth called Lugh, seeking his destiny, sent to Eire by the seagod Manannan MacLir to fulfill an ancient prophecy. With Gilla, a jesting rogue, and Aine, a spirited warrior-woman he came to love, Lugh challenged the Fomor to restore the True King to the throne of Tara, and summoned the Silver Warriors of the Sidhe to fight in the realms of men.