The site is a rath, the Irish term for a ringfort, the most common type of early medieval settlement monument in the country. Thousands were built, typically between the sixth and tenth centuries, as enclosed farmsteads for single families or small kin groups. What makes this particular example quietly interesting is not any exceptional scale but the clarity with which its basic anatomy survives in working farmland.

The enclosure is roughly circular, measuring about 25 metres north to south and 24 metres east to west, and is defined by an earthen scarp still standing around 1.6 metres high and 4 metres wide. Beyond that bank runs an external fosse, a defensive ditch roughly 6 metres wide and nearly a metre deep, though a field boundary to the west has cut across it and interrupted its circuit. The causewayed entrance, a raised crossing point left unexcavated across the ditch, sits at the south-south-east and is generous in width, around 10 metres. Some disturbance on the eastern and south-eastern side suggests possible quarrying at some point, and earlier field boundaries to the south and south-east have since been removed, leaving the interior broadly level. About 520 metres to the west-south-west lies a possible crannog, an artificial or partly artificial island settlement built in a lake or wetland, suggesting that this part of Knockannaveigh may have seen a cluster of early occupation rather than a single isolated farmstead.