What looks at first like a natural undulation in the valley floor is in fact a ringfort, a type of enclosed farmstead or defended settlement built in early medieval Ireland, typically between the sixth and tenth centuries. The enclosure at Aghnahaha is subcircular in plan, measuring roughly 28.5 metres east to west and 24.5 metres north to south, and it sits on a natural elevation that its builders clearly chose with some deliberation.

The site is defined by an earthen bank, now heavily overgrown, that survives in its most complete form along the northern and north-western arc, where it stands nearly two metres high on its outer face, though it drops to a modest 35 centimetres internally. A fosse, the ditch dug to provide the material for the bank and to add an extra line of discouragement to anyone approaching uninvited, runs along the northern side and measures seven metres wide. The entrance, just under two metres across, is also positioned to the north, reached by a causeway crossing the fosse. Around the rest of the circuit the bank has been reduced to little more than a scarp, a low eroded slope, worn down over centuries by agriculture and weather. The dimensions and construction are consistent with a ringfort of middling scale, neither a great fortified enclosure nor a minor farmstead boundary, but something in between, built to last and still quietly present in the valley.