The opening, known as an ope, measures just half a metre wide and fifteen centimetres high, topped with a flat lintel stone, and leads into a souterrain, one of the dry-stone underground passages that were constructed throughout early medieval Ireland, most likely for storage or refuge. What makes this one quietly interesting is that you can read it from both sides: the ope is visible on the exterior of the bank, and a corresponding opening can be seen from the interior, though that inner entrance has been blocked up over time by dead branches.

Souterrains were typically built in close association with ringforts, the circular enclosed settlements that formed the basic unit of rural life in early medieval Ireland. They were usually constructed by corbelling, a technique in which courses of stone are laid so that each projects slightly beyond the one below, creating a rough vault without the need for mortar or a true arch. At Callanafersy, corbelled stonework is still visible within the passage. A depression running approximately four metres by one and a half metres extends south-eastward from the ope into the interior of the ringfort, tracing the buried line of the souterrain beneath the ground. The associated ringfort, recorded separately, sits alongside this feature as part of the same settled landscape.

The site sits within the bank of the ringfort itself, so the ope on the outer face is the most accessible indicator of what lies below. The depression running into the interior gives a useful sense of the souterrain's orientation and extent, though the passage itself is not open for entry.