Hidden in Time by Barbara Longley

Grayce MacCarthy possesses the gift of foretelling—but to her, visions are more a curse than a gift. While visiting Ireland, Grayce takes a hike in the Comeragh Mountains, only to fall into a hidden world with no way out. She’s rescued by a gorgeous warrior dressed like a medieval hero in a romance novel. He’s convinced fate brought her to him, while Grayce is convinced her fae-gifted family will find a way to bring her home.
After more than a millennium, the Tuatha dé Danann magic separating the hidden valleys from the rest of the world is unraveling. A new chieftain must be chosen to guide the clan through the perils to come, and Brían MacSloan is determined to win that coveted role. When a beautiful woman falls into his world, Brían’s life is turned upside down. Grayce challenges him at every turn, questioning his motives, distracting him from his goals, and stealing his heart.
As passion grows between them, Grayce realizes Brían is the only man for her. But then she has a vision revealing a plot to murder Brían, and old rivalries turn deadly. The two are torn apart, and Brían must defeat his enemies and get to Grayce in time, or she’ll be lost to him forever.