The Riders of the Sidhe

A boy, Lugh Lamfada, escapes the destruction of his home by sinister forces. Rescued by sea-god Manannan MacLir, Lugh is sent into Eire on a mission: to aid the Tuatha de Darnann, enslaved by monstrous pirates called Fomor.
Lugh boldly invades the Fomor headquarters, a Tower of Glass, and discovers that the Fomor are the ones who destroyed his home. Through intrigues, battles, and the help of odd, new friends, Lugh challenges the Fomor and their giant, armored leader: Balor One-Eye.
Manannan supplies Lugh with the Riders, a magical warrior troop. In a final confrontation, the Formor hold on the Dananns is broken. Balor and Tower of Glass, however, are still a threat.
The Riders of the Sidhe is Book One of "The Gods of Eire" trilogy, an epic retelling of Celtic legend, filled with fire and magic of the ancient bards.