Liar’s Moon by Kate Sweeney

'In this sequel to Residual moon, Grayson MacCarthaigh stays in Ireland, struggling with her destiny as the true descendant chosen to protect the ancient power of the fabled Tuatha De Danann. Her logical world is now replaced with Celtic legends and myths, goddesses, and immortals. ... someone or something is doing his or her best to see that she fails in her role of protector of the ancients. ... Grayson soon realizes everyone has something to hide. Mendacity runs amok at St. Brigid's Monastery, but all will be revealed under the Liar's Moon '--Page 4 of cover