The machinery exposed two semicircular openings set into the base of a two-metre earthen bank, side by side but distinctly different in scale, and what they lead into is the kind of underground structure that once served early medieval Irish communities across the country.

A souterrain is an earth-cut or stone-lined underground passage or chamber, typically associated with early medieval ringforts and used for storage, refuge, or both. The example at Barrees is earth-cut throughout. The two openings sit roughly 0.8 metres apart on the western side of the forest road. The smaller, southern opening is only about 30 centimetres high and a metre wide, and it gives onto the collapsed upper portion of a chamber that has largely fallen in on itself. The northern opening is more substantial, nearly a metre high and two metres wide, and through it the arched roof of a second chamber is visible, extending westward for about two metres before the passage narrows and slopes downward to the south. Beyond that point, collapsed earth fills and obscures whatever remains. The northwest-facing slope overlooking the valley would have made this a well-concealed location, and the surrounding plantation, planted long after any original use of the site, now keeps it in near-permanent shadow.