The Secret of the Mountain of the Moon is the third book of The Liffey Rivers Irish Dancer Mystery Series. The ancient Irish believed that people were affected by the cycles of the moon and that the moon had real power over the deeds of men. After dancing at an Irish dancing competition (feis) in Beltra, Ireland, 13-year-old Irish dancer Liffey Rivers experiences an extraordinary phenomenon directly above Queen Maeve's cairn in County Sligo, Ireland, on the summit of Knocknarea (the Mountain of the Moon). Soon after this paranormal event, Liffey finds herself seeking answers to life and death questions as she desperately searches on the mountain for a mysterious woman who wears a diamond 'M.'