The earthwork is broadly oval, measuring roughly 24 metres on its north-west to south-east axis and 28.5 metres across, and it is defined by a scarp, essentially a cut or drop in the ground surface, that still stands about a metre high on its north-north-east side, though it tapers to little more than ankle height towards the south. The north-east arc remains well defined, while brambles have colonised much of the south-west, softening what was once a deliberate boundary into something that now looks almost accidental.

Ringforts, known in Irish as raths when earthen, were the most common form of enclosed settlement in early medieval Ireland, typically dating from roughly the sixth to the twelfth century. They served as farmsteads, with the enclosing bank and ditch providing security for livestock and household alike rather than the kind of military defence the word "fort" implies. This one at Culliagh sits on a low rise with open views and a stream or drain running about 50 metres to the north, a pattern consistent with practical early medieval site selection: elevated enough for drainage and visibility, close enough to water for daily use. The interior is largely level, though the south-east quadrant dips slightly. A later field wall cuts straight across the centre on a north-west to south-east line, and two stony heaps survive inside the enclosure, one at the base of a large tree stump in the north-west quadrant, the other beside a hawthorn in the southern half. Both are most likely field clearance heaps, the accumulated debris of farmers working the land long after the rath's original purpose had been forgotten. A separate enclosure sits on a hillock about 170 metres to the south-west, suggesting this corner of Mayo supported more than one focus of early activity within a relatively small area.