Most of these early medieval enclosures, which served as farmsteads and settlement sites across Ireland for roughly a thousand years, were mapped during the nineteenth-century surveys with reasonable thoroughness. The one at Derrynacong in County Mayo slipped through entirely, sitting quietly in the south-west corner of a pasture field on a ridge above a broad stream valley, unrecorded on paper until modern survey work caught up with it.

What survives is a D-shaped enclosure, roughly 26 metres north to south and 23 metres east to west, defined along its curving northern and south-eastern side by a low stony bank. The bank is modest by ringfort standards, only around 0.2 metres high on its interior face, but its builders paid close attention to the natural lie of the land. Because the ridge falls away to the north-north-east, the external face of the bank rises to nearly a metre at that point, compensating for the slope and maintaining a consistent defensive or enclosing profile. On the western side, the straight edge of the D is formed not by an earthwork at all but by a later field wall running roughly north-north-east to south-south-west, and a perpendicular wall clips the southern edge. Whether these walls overlay or simply replaced an original bank on that side is unclear; there is no visible trace of any enclosure beyond them to the west. The interior shows no significant raising above the surrounding ground, as is common in simpler ringforts, and it tilts slightly downward toward the south-west, following the natural contour of the ridge.

The site sits with open views across the stream valley below, a position typical of ringfort placement throughout the west of Ireland, where a commanding outlook over low-lying ground would have been as useful for watching livestock as for any defensive purpose.