A souterrain is an underground passage or chamber built from drystone, typically during the early medieval period in Ireland, and associated with nearby settlement enclosures. They are thought to have served as places of refuge, cool storage for foodstuffs, or both, though their exact function is still debated. The cashel itself, a circular stone-walled enclosure of the kind found across the west of Ireland, provides the immediate context: this souterrain sits inside its walls, rather than beneath a dwelling, which already sets it apart from more familiar examples.

The structure is L-shaped and runs to more than ten and a half metres in total length. Entry is made at the south-eastern end of the first chamber, which measures roughly five metres long, just under a metre and a half wide, and a little over a metre and a quarter high; a tight, deliberate space. From the north end of that chamber's east wall, a short connecting passageway, less than two metres long and only seventy centimetres wide, leads into a second chamber running in a different direction, north-east to south-west, and marginally taller at around one and a half metres. What makes this souterrain particularly notable is the presence of alcoves cut into the walls at several points: in the north and east walls of the first chamber, in the south wall of the passageway, and in the north wall of the second chamber. These small recesses may have held lamps, objects of significance, or provisions, though the stonework itself does not say.