The Lost Witch by Paige Crutcher

Paige Crutcher, author of the "mystical, magical, and wildly original"* The Orphan Witch, weaves a spellbinding tale of contemporary fantasy, romance, and Irish mythology as a woman torn out of time faces a dangerous coven, otherworldly creatures, and a seductive demi-god to save the souls of a cursed island in The Lost Witch.
1922. The town of Evermore off the coast of Ireland is under the protection of a Goddess. She has bestowed power upon village healer Brigid Heron to ensure the heart of magic within the Lough of Brionglóid—the lake of dreams—remains untouched. For the witches of Knight want to absorb its powerful energies and release the Damned from the Otherworld. Brigid has devoted her whole life to being Evermore's guardian, immersing herself in witchcraft, and sacrificing her own dreams. Until Luc Knightly, a trickster god with his own claim on the lough, offers Brigid her heart's desire in exchange for betraying her Goddess's...