A souterrain, to use the proper term, is an underground passage or chamber associated with early medieval settlement in Ireland, typically thought to have served as a place of refuge, storage, or concealment. This one at Ballynacarriga was not alone; a second souterrain was excavated roughly twenty metres to the west, within the same enclosure.

The excavators, led by Noonan and colleagues and publishing their findings in 2004, recorded two passages connected by a creepway. The first ran northwest to southeast, measuring seven metres in length, 1.2 metres wide, and about 0.8 metres high, with vertical sides and rounded, steeply sloping ends. Near its northwest end, an earth-cut step was identified as the probable entrance. The second passage, shorter at 5.2 metres and lower at a maximum height of 0.75 metres, joined the first near its southeast end and then turned a further 3.7 metres to the southeast, with the excavators suggesting this trailing terminus may have served as an exit. Charcoal fragments appeared in two of the deposit layers within the passages, hinting at burnt timber linings or structural supports, and the inverted V-shape of the creepway ceiling suggests that timber planking once roofed it, the organic material long gone but its form preserved in the earth above. A comparable pattern of charcoal was found in the nearby western souterrain, making timber lining plausible for both, though no definitive structural evidence survived in either case.