Alongside the remains of a stone hut, a souterrain, an underground passage built from dry-laid stone without mortar, runs for four and a half metres into the earth, ending directly below the hut itself. These passages are found across early medieval Ireland, typically associated with settlement sites, and were likely used for storage or refuge. This one is narrow enough to demand a crouch: less than a metre in both height and width, its walls leaning slightly inward in a technique known as corbelling, closing toward a roof of flat lintels.

The site was recorded by the antiquarian T. J. Westropp in 1912, who classified it as a fort, though the structural evidence points instead to a domestic settlement. What makes the Cool souterrain particularly interesting is a detail from its floor. Limpet shells and other debris had apparently fallen through from the hut above into the passage below over time, and when these were radiocarbon dated, they returned a result of 930 plus or minus 80 years before present, placing occupation somewhere around the tenth or eleventh century. That a scattering of shellfish debris could anchor a site so precisely in early medieval Kerry gives the place a quietly remarkable precision that grander monuments rarely manage.