The Lady of the Sea by Rosalind Miles

Isolde, heir to the throne of the queens, is now a sovereign in her own right. With the glories of the throne comes the responsibility of a queen, and Isolde knows she must return to her beloved Western Isle. She can no longer tolerate her marriage to King Mark of Cornwall, a marriage she has accepted for years in order to save her country from the threat of war and to be near her only love, Mark's nephew Tristan of Lyonesse. King Mark, always cowardly and spiteful, is too heavily influenced by his monks and counselors, who loathe the powerful and independent Isolde. And so she leaves Cornwall for good and comes home to Ireland, where her lords face a growing threat from the warlike Picti, who live in the barren highlands to the north of England. The Picti have a bold new king, Darath, who is determined to take the riches of Ireland for his own people, whether by war or by marriage with Isolde.

Isolde gathers her armies to confront the Picti and faces a violent conflict as well with King Mark, who vows he will not let a prize like Isolde, and Ireland, slip from his grasp. Isolde is last in a line of famous warrior queens who have guarded Ireland from time before memory, and now she-and her knight, Tristan-must play out their fate and face her enemies in a final battle, a war that could spell ruin for them both.

This book is part of the "Tristan and Isolde" series.
Here are some other books from this series: