This looks like the underground dwelling of the Tuatha de Danaan, or "Children of Danu", the mythological mother goddess of the Irish Celts. Creatures such as leprechauns and other Fairy Folk were known to inhabit these places, particularly under the burial mounds, which were also known as the fairy mounds for that reason.
Lars Van De Goor is an experienced photographer from Netherlands that prefers to shot trees and ancient forest in an unusual way. Lars was recently nominated for an Hasselblad Master Awards at the Landscapes/Nature Category. We wish him good luck!