The Lost Girl King by Catherine Doyle

Amy and Liam Bell have been packed off to stay at Gran’s house in the wilds of Connemara for the summer. Out for a walk on the first morning of their holiday, they trace the flight of a hawk to a nearby waterfall – only to watch the bird disappear through it. Intrigued, the children follow and soon realise they’ve discovered the entrance to T?r na n?g, the legendary land of eternal youth. But they’ve been tricked. Almost immediately Liam is captured by a troop of headless horsemen who take him to Tarlock, the ruling sorcerer of T?r na n?g, who is seeking the bones of a human child for a sinister new spell.