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.
Mystical places - as mentioned before - we dream of these in order to escape & find true meaning & mystery. Some believe that a mystical place can also be a representation of the Higher Self or Godhead trying to get our attention.
The most beautiful sacred places Vortexes - Arizona Sedona - long known of people who are interested in treatment, mysticism and metaphysics coming to see the beauty of dramatic red rocks. The region, however, is known for its vortexes - power centers of the kinetic energy which appreciably affect those who attend them and meditate.