The structure is a souterrain, a type of underground passage or series of chambers built without mortar, using carefully laid dry stone, and associated in Ireland with early medieval settlement. This one sits inside a rath, an enclosed farmstead of the same period typically defined by a circular earthen bank and ditch. What makes Rockfield quietly remarkable is the gap between what has been documented and what local memory insists is there.

Three chambers are currently known, arranged roughly north to south and connected by creep-ways, those low, deliberately tight passages, often only a few centimetres wider than a person, which would have forced anyone entering to slow down considerably. The northernmost chamber has largely collapsed and its lintel roofing has been removed, but it still provides the current point of entry. From there, a lintelled opening at floor level leads through the inner bank of the rath into a second chamber measuring 4.8 metres in length and over two metres in height, its walls corbelled inward above a metre to carry a roof of large overlapping stone slabs. A wedge-shaped creep-way at the far end, narrowing as it goes, opens into a third chamber some seven metres long, whose southern end is now filled with soil and scattered stone. At least one lateral shaft extends from the third chamber's north wall before becoming blocked with soil, and there are hints of further openings at the obscured south wall. The structure was already recorded on Ordnance Survey maps by 1917. Local tradition, however, holds that the souterrain has up to six interconnected chambers in total, with a second entrance, now blocked, somewhere in the field to the south-east. Surface collapses visible within the rath interior, where large stones break through the turf, lend that tradition a degree of credibility, suggesting the underground network continues both westward and through the centre of the enclosure toward its eastern bank.