Power of Eternity by Carly Spade

In the year 1000, an ancient warrior Druid is punished for his arrogance and insubordination. His punishment: He will one day be compelled to save a life at the cost of giving up part of his power. Over a thousand years later, he will discover the true meaning of immortality.
Abby Weber and her best friend Phoebe arrive in Ireland, ready for their dream vacation. Their plan is simple. Drink pints of Guinness, see a few castles, and enjoy the splendors of the Emerald Isle. Meeting a cocky, bad boy Irishman named Erimon, who irritates her as often as he makes her stomach flutter, is not part of the plan. Neither is being chosen to die by the Dullahan, a Celtic death god. Fortunately for her, the bad boy Irishman is an immortal warrior Druid.
Abby reluctantly accepts that only the infuriating Druid can help her fend off the Dullahan. Erimon enlists the help of a mismatched band of mythical friends, and together they make a new plan: Kill the Dullahan before it kills her. Diving into a magic world she never knew existed, fighting an increasing heat rising between her and Erimon, and relying on friendships forged in battle, Abby will discover an entire other side to herself. An untapped fierceness ready to challenge death.