It was found not by archaeologists but by a farmer digging potatoes, who broke through into something unexpected below the ground. The opening was sealed shortly afterwards, judged too dangerous to leave accessible to livestock, and the earth quietly closed over it again. No surface feature marks the spot.

What the farmer uncovered was almost certainly a souterrain, an underground stone-built passage or chamber constructed during the early medieval period, typically between roughly the seventh and twelfth centuries. Souterrains were built by hand, usually using dry-stone walling and large flat capstones laid across the top as a lintelled roof, which is exactly how the Ballycasey entrance was described by those who saw it. The interior was reported as square in shape, which is slightly unusual; many souterrains are elongated passages, though square or roughly rectangular chambers do occur, sometimes as terminal rooms off a longer tunnel. Their original purpose is not fully agreed upon, but likely uses include cool storage for dairy produce, temporary refuge, or both. This one sits on a south-facing slope, a detail that might once have mattered to whoever chose the site, whether for drainage, orientation, or proximity to a settlement that has since left no trace of its own.