The gap it left, roughly sixty by seventy centimetres, is just wide enough to peer through, and what it exposes is a corbelled chamber built from limestone rubble, the stones ranging from ten to thirty centimetres across, with walls angled inward and upward in the dry-stone technique that gives souterrains, early medieval underground passages and chambers, their characteristic vault. The accessible chamber measures roughly three metres by nearly two, and stands one and three-quarter metres high. Beyond it, further chambers stretch away in both directions, most of them blocked by accumulated earth and fallen stone.

A souterrain of this kind would typically have been associated with a nearby settlement, and sure enough the remains of a levelled ringfort lie around thirty to forty metres to the south-west. Ringforts, the enclosed farmsteads of early medieval Ireland, often had souterrains attached, used variously for storage, refuge, or both. At Neddans the underground complex appears to be unusually extensive. There are at least four chambers in total, connected by low lintelled passages, some blocked, some only glimpsable. The north-east chamber, which can be seen but not entered, is roofed by large lintels more than one and a half metres long and has its own apparent entrance leading to yet another passage beyond. Local people who explored the souterrain in earlier years reported that at least one chamber was tall enough for a person to stand upright without reaching the ceiling, which tallies with the scale suggested by the surviving stonework.

The site sits on an east-facing slope in undulating farmland, and the structure is now largely filled with washed-in debris. The entrance passages are either blocked or extremely narrow, the widest apparent entry being sixty-five centimetres across and only forty-five centimetres high, which gives a reasonable sense of what access would have demanded even in its original state. The visible chamber is the only one that has been properly documented; what lies further along the interconnecting passages remains a matter of local memory and educated inference.