The passage runs roughly five metres from north-northwest to south-southeast, widening slightly as it goes, from about a metre across at its southern end to one and a half metres at the north, and dropping to a depth of somewhere between half a metre and just under a metre. It is open to the sky now, but the form it describes is that of a souterrain, an underground passage or chamber built in early medieval Ireland, most often associated with ringforts and cashels, and thought to have served for storage, refuge, or both.

The cashel within which this souterrain sits is a stone-walled enclosure, the word cashel deriving from the Irish caiseal, referring to a dry-stone fortification of roughly circular plan. Such structures were typically built in the early medieval period, broadly between the fifth and twelfth centuries, and the presence of a souterrain in the northwest quadrant of one is entirely in keeping with the pattern found at similar sites across Munster and beyond. The undulating, now forested landscape of Erinagh More would have looked very different to whoever constructed these features, probably open farmland organised around the enclosed settlement that the cashel defined. The souterrain itself survives as an open section of passage rather than a roofed or partially buried one, which suggests either that its covering stones were removed at some point or that collapse has long since exposed the interior.