What makes Corgreagh unusual is not simply its age but its engineering: two banks, two ditches, and a carefully preserved causeway entrance, all arranged with a precision that suggests this was not a casual enclosure but something built with serious intent.

The site is roughly circular, with the inner area measuring about 39 metres across from east to west. That interior is bounded by an earthen bank, still rising to around three metres on its outer face, which is separated from a second, outer bank by a water-logged fosse, the ditch-like depression that would have served as both drainage and obstacle. A fosse of this kind is a defining feature of ringforts, the enclosed homesteads that were the dominant settlement form in early medieval Ireland, typically occupied between roughly the fifth and twelfth centuries. At Corgreagh the outer bank is itself substantial, with a base width of nearly eight metres in places, and beyond it runs a further shallow outer fosse or drain. The maximum external diameter, taking in all these layers, reaches about 58 metres east to west. The entrance is at the south-east, where a causeway, a metre high and two metres wide at its top, carries the approach across the inner fosse through gaps in both the inner and outer banks. The alignment of the whole structure on the end of a natural spur would have given its original occupants clear sightlines across the surrounding landscape while the topography itself reinforced the earthworks.

The site is overgrown, with bushes colonising the banks and the inner fosse remaining waterlogged, but the essential form of the monument is well preserved. The causewayed entrance at the south-east is the clearest point of orientation for anyone reading the earthworks on the ground.