The disturbance was enough to shift two of the roof lintels and partially collapse the walling, announcing the presence of something far older than the forestry work above.

A souterrain is a deliberately constructed underground passage or chamber, typically built during the early medieval period in Ireland and used for storage, refuge, or both. The Oughtihery example was investigated in 1970 by D.C. Twohig, who recorded a single wedge-shaped chamber built from stone, measuring roughly 3.15 metres in length, with a maximum width of 1.26 metres and a height of 1.5 metres. The roof lintels rest on corbels, stone blocks that project inward from the walls to carry the weight above. What makes the structure particularly intriguing is what Twohig found at its southern end: an opening at floor level leading into a low passage about a metre long. This passage was inaccessible at the time of investigation, and Twohig suggested it could be either a creephole connecting to a second chamber beyond, or the original entrance to the souterrain itself. That question, as far as the record shows, remains open.