The opening is uncomfortably small, even by the standards of these structures: a lintelled gap just 65 centimetres wide and 35 centimetres high, reached by dropping steeply down a pit roughly a metre deep inside the fort's northern bank. A souterrain is an underground stone-built passage or chamber, typically associated with early medieval ringforts in Ireland, and thought to have served as a place of refuge, storage, or concealment. This one follows a northwest to southeast orientation before curving southward at its collapsed end, roofed along its explored length with flat capstones.

McCarthy, writing in 1977, was able to investigate part of the passage but could not reach its terminus. The entrance area, it appears, had already been compromised before any formal investigation took place, most likely during tillage of the land above, with subsequent interference making matters worse. More intriguing is a rectangular depression near the centre of the fort, measuring roughly 4.5 metres by 3.9 metres and sunk about 30 centimetres into the ground. McCarthy proposed that this hollow may mark the position of a chamber that was once accessible from the southern end of the passage, now blocked. If so, the souterrain was more elaborate than its surviving remains suggest, with at least one additional space branching off the main tunnel.