During archaeological excavations carried out in 2001 ahead of the construction of the N25 Youghal Bypass in County Cork, diggers uncovered a souterrain, the Irish term for a man-made underground passage or chamber system typically associated with early medieval ringforts. These structures were cut into the earth or built from stone, and were used variously for storage, refuge, or ventilation of adjacent buildings. What emerged at Ballynacarriga was not a simple tunnel but a four-chambered system arranged in a rough horseshoe shape, sitting within the south-western quadrant of a larger enclosure and to the north of a circular structure.

The geometry of the place is oddly precise for something carved from earth. The easternmost chamber ran roughly six metres north to south, with sharp vertical sides and a flat base, and connected at its northern end to a narrower chamber running east to west. That second chamber widened and deepened as it turned into the western arm of the horseshoe, where two further chambers were found. The largest of these, in the west, reached a height of 1.8 metres, tall enough to stand in, and was connected back to the eastern arm by a narrow creepway, the kind of low, tight passage that would have slowed any unwanted intruder considerably. A shelf cut into the western wall of this chamber may have served as a secondary exit. The fourth and southernmost chamber tapered to just 0.53 metres in height at its far end, and the excavators concluded that this constricted point was most likely the original entrance. A second souterrain was found roughly twenty metres to the east during the same excavation, suggesting the enclosure was a more substantial settlement than the bypass corridor alone could reveal. Whether the chambers were originally lined with timber or stone remains uncertain; no timber survived, and if stone was ever used, it had long since been taken for use elsewhere.