The Shamrock Shore (Heroes of the Misty Isles #3) by Sandy Dengler

It's a raw spring day in fifth-century Ireland when fugitive Padraic of Lambay is rescued from drowning by a farm girl named Bridey. Padraic, though resourceful and clever, has come to the end of his strength. He cannot elude the man on the black horse who has pursued him for the last year. Yet Bridey, a mere slip of a girl, helps him escape once more. Together he and Bridey set out on a pilgrimage -- Padraic to find his freedom and Bridey to discover the secrets of her past.