Not a gate so much as a deliberate puzzle, and one that has lost at least one of its original framing stones to the sea. The fort occupies the tip of a triangular, north-west-facing promontory on the south-east shore of Achill Island, surrounded on all sides by sea rocks and cliffs, with the Atlantic Drive road visible above it to the north and east. The interior, roughly 50 metres long by 28 metres wide and nearly level, is entirely featureless today; the sea rocks extending from the promontory's edge suggest the enclosed area was once considerably larger before erosion claimed the margins.

A promontory fort is essentially a headland where a natural defensive position, cliffs on most sides, is completed by an artificial barrier across the landward neck. At Dun Bunnafahy, that barrier is a straight bank of earth and stone, up to 12 metres wide at its base and rising to 2.3 metres at the northern cliff edge, originally topped by a stone wall of roughly 1.85 metres in width. The wall survives in places to five courses of regular schist slabs, visible along both inner and outer faces. In front of the bank runs a deep ditch, flat-based and steep-sided towards the north, with the ditch floor lying 6 metres below the top of the bank and 3 metres below the outer ground level. A causeway 5 metres wide crosses the ditch near the centre, its sides showing traces of stone revetment. Within the wall near the centre sits a small square cell-like structure, just 1.26 metres across and formed from upright slabs; whether it opened inward or outward remains unclear, but its position opposite the causeway suggests it was part of the entrance sequence. The antiquarian Thomas Westropp noted the site in 1914, recording two large upright slabs flanking a probable entrance at the southern end of the bank, along with a circular hut at the causeway head. The hut has entirely vanished, and one of the uprights has since gone over the cliff.

The fort is visible from the Atlantic Drive, and a modern path now runs along the cliff edge at the southern side of the site. An earthen bank running roughly parallel to the main defences, about 25 metres to the east, represents a later cliff-edge wall, suggesting the promontory continued to serve some boundary or defensive purpose well after the original fort was built. The whole setting, mountainous bog, sheep grazing, the long coastal views, gives the place an atmosphere of considerable remoteness even when the road is only a short distance away.