A cashel is a ringfort built from stone rather than earthen banks, typically enclosing a farmstead or small settlement, and this one at Barnacahoge once formed a subcircular enclosure somewhere between twenty-five and thirty metres across. What makes it quietly arresting is not the wall itself, which survives only as an eighteen-metre arc of sod-covered rubble rising no more than eighty centimetres above the ground, but what lies beneath the interior: a souterrain, an underground stone-built passage or chamber used in the early medieval period for storage or concealment, sitting roughly at the centre of what was once a complete enclosure.

The cashel was already mapped by 1838, when the first Ordnance Survey six-inch sheets recorded it as a distinct subcircular feature with a cluster of vernacular houses immediately to its south. Later map editions tell a different story: the enclosure had by then become absorbed into a network of small, irregular walled fields, its form fragmented and obscured. Aerial photography captured the site at an earlier stage of this process, showing a wall running across part of the north-west quadrant of the interior and a possible field clearance cairn pushed up against the cashel wall at the south. That gradual domestication of the monument eventually reached its conclusion when land reclamation destroyed the western two thirds of the cashel entirely. The remnant that survives at the north-east to south-east is evident mainly as a jumble of loose stone, with sections of the internal wall face still intact in places, and field clearance stones heaped on top where farmers continued the slow work of tidying the land long after the original structure had lost its meaning. An old farm trackway, flanked by field walls, runs along the eastern edge of the site, a reminder that this particular patch of Mayo stony ground has been worked and reworked across very different centuries.