A souterrain is an underground stone-built passage or chamber, constructed during the early medieval period in Ireland, typically associated with nearby settlement and used variously for storage, refuge, or concealment. What emerged here was not a dramatic discovery but a fragmentary one: a single rectangular chamber, barely a metre long and less than a metre wide, with a low creepway at its eastern end that may once have served as the original entrance. The whole structure is roofed with flat lintel stones, and the creepway, at just thirty centimetres high, would have required a person to crawl flat to pass through it.

The site was investigated by Cleary in 1987 and was already in a ruinous and dangerous condition by that point. Only one chamber could be surveyed, though the investigator was confident that further chambers extend to the west, still unexcavated and unrecorded. More intriguing still, Cleary noted what he described as surface irregularities and low levelled banks on the southern side of the souterrain. These features are suggestive of a ringfort in the vicinity, a type of enclosed farmstead common in early medieval Ireland, typically defined by one or more earthen or stone banks. Souterrains are frequently found in association with ringforts, built beneath or adjacent to them, which gives the Carrigeen site a wider context even if much of it remains buried and unconfirmed.