Heart of the Sidhe by Kenneth C. Flint
The Tower of Glass has been destroyed. Balor of the Evil Eye is dead. But now the Tuatha de Danann clans must face a new threat from the piratical Fomor race. While one faction tries to make a peace, a ruthless military under High-Admiral Tharkus forms a conspiracy to thwart the attempt, gather their forces, and strike back at the Children of Danu. The young hero Lugh of the Long Arm must gain new strength for his people by seeking out the lost source of the Sidhe power.
He is aided in his quest by the giant Dagda, the shape shifting Púca, the raven-woman Morrigan, his beloved Aine, and her father, the disguised sea-god Manannan MacLir. Against them are arrayed the powerful and brutal forces of the Fomor, utilizing their ancient scientific and mechanical powers to defeat the magic of Tir-na-nog, seize the Heart of the Sidhe, and again enslave the de Danann people.
This new novel of the Sidhe series is, like the rest, an epic fantasy for all ages, blending the myths and legends of Celtic Ireland with her history in a tale of high-adventure and romance.