On elevated ground along the northern bank of the Blackwater river in south Kerry, within a plantation of deciduous trees, a ringfort that once appeared as a clear circular enclosure on Ordnance Survey maps has been almost entirely removed. A large section of the ground has been dug into at the south-east, and where a rectangular structure once stood at the centre of the enclosure, there is now only a subrectangular depression measuring 3.6 metres north to south, 2.9 metres east to west, and up to 1.4 metres deep, filled with rubbish.

The Ordnance Survey Name Books, compiled in the nineteenth century, recorded a "fort and cave" at this location, along with traces of an old ruin. That reference to a cave is suggestive. Close to the southern edge of the depression, a small opening measuring roughly half a metre wide and thirty centimetres high leads down about seventy centimetres into the ground, and may mark the entrance to a souterrain, an underground stone-lined passage typically associated with early medieval ringforts and used for storage or concealment. Two further openings to the north-west and north-east of the depression may form part of the same underground system. Ringforts, or raths, were the most common form of rural settlement in early medieval Ireland, and those with souterrains attached were generally of some local significance. At Knockaunnahona, the ground itself seems to retain that underground complexity even as everything above it has been erased.