The souterrain, a type of man-made underground structure built during the early medieval period in Ireland, often for storage or refuge, is part of the cashel known as Caher Murphy, or Cathair Mhurfaí in Irish. What makes the arrangement genuinely unusual is how completely the underground world here is nested within the above-ground one: the entrance lies at the north-west side of one of the clocháns, the dry-stone beehive huts that cluster together inside the oval cashel wall, and the passage then burrows westward before the chamber opens beneath the cashel wall itself.

The souterrain is L-shaped, with a passage running some 4.65 metres in a west-south-westerly direction, roofed by large flat slabs and standing no more than 1.2 metres high and 0.95 metres wide, so any exploration would require some determined crouching. The chamber at its end is accessed through an opening barely 0.4 metres wide and 0.7 metres high, with a low sill stone across the threshold; once inside, the space opens to 3.6 metres north to south and reaches a maximum height of 1.5 metres. Two air-vents pierce the south wall at roughly 1.2 metres above ground level, and the roof of the chamber is carried on projecting corbels supporting a single large capstone. Five clocháns and a sixth irregularly shaped structure occupy much of the cashel interior, making Caher Murphy one of the more complete examples of its kind on the Dingle Peninsula. The Office of Public Works carried out considerable restoration work on the site, and during nineteenth-century work there an elaborate cross-slab was uncovered; it is now held in the National Museum of Ireland. A fragment of a rotary quern stone, used for grinding grain, was also found lying loose within the cashel walls.