A souterrain, to use the correct term, is an underground stone-lined passage or chamber associated with early medieval ringforts, likely used for storage, refuge, or both. The one at Barrettstown is only a possibility, noted in the northeast quadrant of the enclosure, but its suspected presence adds a layer of intrigue to what is already a quietly complicated site.

The ringfort itself is oval in plan, measuring roughly 37 metres north to south and 34 metres east to west. A ringfort, or rath, is a type of enclosed farmstead typical of early medieval Ireland, usually dating from roughly the fifth to the twelfth century, defined by one or more earthen banks and ditches. At Barrettstown, much of the enclosing earthwork survives as a scarp, a steep-sided cut in the ground rather than a raised bank, though a low bank does remain along the northern to northeastern arc. That surviving bank, while worn down, still shows measurable internal and external faces. The outer fosse, a rock-cut or earthen ditch running around the outside, is about 8.75 metres wide at its broadest and deepens to around 0.75 metres; it narrows as it approaches the east and south, where later field boundaries have cut across it. The southern and southeastern stretch of the fosse is waterlogged, which gives some sense of how the drainage of this gentle slope behaves across the seasons. On the western side, sheep erosion has worn away a short arc of the scarp. An entrance is likely situated at the south-southwest, where a gap of roughly five metres in the earthworks suggests a causeway crossing the fosse. Cahill noted the possible souterrain in 1982, and it remains unexcavated as far as the available record indicates.

The interior slopes gently southward following the natural lie of the land, and the whole thing sits unobtrusively in the southeast corner of a field still in agricultural use. The fosse, the worn bank, and the probable entrance are all readable on the ground if you know what you are looking for, though the sheep have done their work on the western edge.