The hollow is roughly subrectangular in shape, measuring about nine metres along its longer axis and nearly four metres across, and sinking to a depth of around sixty centimetres. Nothing about it announces itself dramatically, which is precisely what makes it interesting.

A souterrain is a man-made underground structure, typically a stone-lined tunnel or chamber, built during the early medieval period and usually associated with ringforts, the circular enclosed settlements that were the dominant form of rural habitation in early medieval Ireland. Souterrains served variously as places of refuge, storage for dairy produce, or both, and their construction required the removal and disposal of considerable quantities of earth and stone. What is telling at Porterstown is the absence of any up-cast, meaning no visible mound of excavated material nearby. This absence points away from a deliberately dug pit and towards a roof collapse, suggesting that the hollow is what is left after the covering structure of a souterrain gave way, leaving the depression we see today. The ringfort it sits within is a separately recorded monument, and the souterrain occupies its south-south-west quadrant, tucked into the slope of the hillside.