What lay below was not a void but a souterrain, an underground structure of the kind built across early medieval Ireland, typically by cutting chambers into earth or rock and roofing them with corbelled stone or, as here, with domed earth. They served various purposes, among them storage, refuge, and the cool preservation of perishables. The Kilnagurteen example, however, is unusually extensive, and its accidental discovery under a new suburban street gives it a particular strangeness.

The souterrain consists of six roughly oval, earth-cut chambers arranged in two offset rows, the three westerly chambers running on a slightly different alignment to the three easterly ones. They are connected by creepways, low narrow passages that require a person to crouch or crawl, which is a common defensive feature of such structures, slowing any intruder trying to move quickly through the dark. The largest of the six, chamber 1, stretches 5.6 metres in length and varies between 1.7 and 3 metres in width. The smallest, chamber 3, is the one whose roof collapsed in 2001, opening the site to the light for the first time in centuries. Each chamber also retains blocked-up construction shafts, vertical openings used during building to remove excavated earth and then sealed once the work was complete. In total the network is a considerable feat of underground engineering, carried out without any of the machinery that inadvertently revealed it. Following archaeological investigation by McCarthy in 2001, the collapsed entrance was closed again, this time with reinforced concrete slabs.