Duncan by Hazel Hunter

A modern day heiress uses all her wiles to escape Medieval Scotland.
Although Nicole Fairburn will one day inherit the estate of one of the richest men in the world, his wealth has forced her to always be on guard. But even her wariness can’t protect her from those closest to her.
Although the latest arrival to Dun Ard seems unflustered by time travel, Duncan suspects that the cool young lady is hiding something. At the same time, too much about her seems familiar and entirely enchanting.
Unfortunately there is no time to sort out feelings. Enemies from within and without the castle besiege the clan. Only then do Duncan and Nicole understand how far they must go to save those they love.