The Last of the High Kings by Kate Thompson

Traveling to the land of eternal youth was the only way J.J. Liddy could stop time from leaking from his world to T'ir na n'Og. But fifteen years after returning from the land of the faeries, J.J. wonders if that long-ago visit is responsible for the strange things now happening to those around him.

Why does his daughter Jenny roam barefoot through the wilds, when she should be in school? When did the mysterious white goat begin to patrol the hillside? What is the secret project that J.J.'s son Donal is attempting? And who is the ghost guarding the stone beacon at the top of the mountain—and why has Jenny befriended him?

Finding answers to these questions will take J.J. and his family on the most important and dangerous journey of their lives. If they fail, it will undo all the good that J.J. accomplished fifteen years ago. But if they succeed, they will defeat the forces that are gathering to destroy all of mankind, and finally secure the future of the last of the high kings.