What survives on the headland at Knockaun, on the north Mayo cliffs near Downpatrick Head, is essentially a single drystone barrier running across the neck of a coastal promontory, cutting off the headland from the flat bogland behind it. This is what archaeologists call a promontory fort, a type of defensive enclosure that uses natural cliff edges as its walls on most sides, with a constructed barrier closing off the only landward approach. The Knockaun example stretches 77 metres across that approach, built from sandstone slabs with a rubble core, and still stands to a height of 2.35 metres near its eastern end, which is considerable given its condition elsewhere.

The wall's outer face is gently battered, meaning it slopes slightly inward as it rises, a technique that adds structural stability. About 33 metres from the western cliff edge, a stone-lined gap roughly a metre wide may mark the original entrance. A small stone building, measuring roughly 2.3 by 2.5 metres, was constructed against the outer face at some point and used as a sheep shelter, with traces of two more similar structures nearby, though whether any of them have a connection to the fort's original use is unclear. When the antiquarian Thomas Johnson Westropp recorded the site in 1912, he described an earthen bank as well, but no such bank survives; the grassed-over collapse lying along the wall's footings may be what he was looking at. Aerial photographs have revealed a low L-shaped bank running through the interior, but at ground level the enclosed area offers little to read. Some faint suggestions of circular hut sites along the northern edge are noted but not convincing. The wider landscape compensates for the interior's silence: 190 metres to the west lies Dunbriste, the famous sea stack that was once a continuation of Downpatrick Head itself, and several Early Christian monuments along with two ring barrows, a type of burial mound encircled by a ditch, are visible in the surrounding area.