The lost colony Artemis Fowl Series, Book 5

Summary: Ten thousand years ago, humans and fairies fought a great battle for the magical island of Ireland. When it became clear to the fairy families that they could never win, they decided to move their civilization underground and keep themselves hidden from the humans. All the fairy families agreed on this, except the 8th family, the demons. The demons planned to lift their small island out of time until they had regrouped and were ready to wage war on the humans once more. However the time spell went wrong, and the island of Hybras was catapulted into Limbo, where it has remained for ten thousand years.