In ancient times it was considered an entrance to the underworld of the Sí - the fairy people.  Rathcroghan is a very ancient landscape with many barrows, ring forts and other structures. It is also the reputed domain of Queen Méabh and the starting point of An Tain Bó Cuailgne.

While the cave is a natural structure, it seems to have been modified in past times and reinforced with slabs of limestone. This includes an ogham stone that was placed into the roof. Of itself the stone has nothing to do with the cave (it is actually a grave marker that was moved here) though it certainly adds some mystery. 

The cave is about 20 metres deep and 50 metres long. Quite a simple passage - wet and with a large chamber. 

When I was there I saw the remains of some candles and crow feathers stuck into the walls. Looks like some crusty folk were perhaps conducting secret rituals down below.