The entrance to this underground structure, measuring just one metre wide and 0.7 metres high, opens directly into a stone chamber that would have required visitors to crouch or crawl through its confined spaces. This initial chamber, running southwest to northeast, features a small recess in its eastern wall; perhaps once used for storage or as a lamp niche. The drystone walls curve naturally with the rock face, whilst overhead, stone slabs form a solid roof that has endured for centuries.

Beyond the first chamber, a narrow doorway barely 61 centimetres high leads through a short passage into a second chamber oriented northwest to southeast. At its northwestern end, tantalisingly, lies a blocked entrance to what local knowledge suggests is the largest chamber of all, though it remains unexplored and inaccessible. The entire structure demonstrates the remarkable engineering skills of its builders, who worked with the natural rock floor and constructed walls without mortar, creating a watertight underground refuge that has survived intact despite the accumulation of mud over its floors.

The souterrain has yielded at least one intriguing artefact: a perforated bead crafted from a horse’s tooth, now housed in the National Museum of Ireland. Such souterrains, found throughout Ireland but particularly common in Ulster, likely served multiple purposes; as places of refuge during raids, storage facilities for valuable goods, or perhaps holding ritual significance for the communities that built them. The Tonregee example, positioned strategically along an east-west ridge, offers a glimpse into the ingenuity and resilience of early medieval Irish society.