The Last of the High Kings (The New Policeman Trilogy)

JJ Liddy sometimes blames his unreliable temperament on the visit he made to Tir na n'Og, the land of eternal youth, when he was fifteen years old. It's perhaps not surprising that his children have also turned out to be a little eccentric, especially eleven-year-old Jenny. She forgets to go to school, can't bear to wear shoes, and spends entire days roaming the mountainside.

It's up there that she meets the ghost. He is guarding a pile of rocks known as the beacon, and when some archaeologists arrive to excavate it, they run into the strangest kind of obstruction. But it is not people the ghost fears, and when the real enemy finally reveals itself, the future of the entire human race is threatened. Only Aengus Og and his fairy kin can help now.