When the machine lifted one of the large flat sandstone lintels from the south-western corner, it exposed a souterrain, an underground stone-lined passage or chamber built in early medieval Ireland, typically for storage or refuge. The structure lay just inland from the sand-dunes near the seashore at Castlegregory, covered by roughly one and a half metres of light sandy topsoil laced with scattered sea shells. Once the opening was made, local people reported finding fragments of bone and shell inside, though these most likely tumbled in from the collapsed topsoil rather than representing any deliberate deposit.

The chamber was modest but carefully made. Its walls were built of small, neatly coursed sea-rolled stones, presumably gathered from the nearby shore, and the roof was carried on six large flat sandstone lintels. The single chamber ran on a north-east to south-west axis, between 0.8 and 0.95 metres wide and about one metre high, with a confirmed length of at least three metres. The north-eastern end was entirely blocked by ancient collapse, so the full original extent was never established, and the true entrance may well have lain somewhere beyond that blocked section. A small break in the north wall at ground level could have been a deliberate alcove or cavity. The south-western end wall was curved rather than straight, a feature noted as typical of souterrains across the Dingle Peninsula. The structure was inspected shortly after discovery by Hurley of the Office of Public Works, and the description was later published in J. Cuppage's 1986 archaeological survey of the Dingle Peninsula. It is now inaccessible.