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.
Gary McParland ENTWINED • THE DARK HEDGES, ARMOY, CO ANTRIM The Dark Hedges is an avenue of 300 year old beech trees situated along Bregagh Road 3 miles from the village of Stranocum. They are a tourist attraction and are photographed regularly.
The babe laughed a laugh that filled all the hollow spaces in the forest. As I fairy I smiled and helped carry it to the tree. Any abandoned or orphaned creatures we brought to our sanctuary. The head fairy, my mother, was the in charge and each fairy was assigned a creature. I was lucky and got the first human. (credit to @Wizard003)