It is a rath, the Irish term for a ringfort, and its geometry is precise enough to read clearly even after more than a thousand years of farming around it. The interior is level and featureless now, forty metres across, offering no immediate clue as to what once stood inside, but the engineering of its boundaries is still legible in some detail.

Ringforts were enclosed farmsteads, typically built during the early medieval period, roughly between the fifth and twelfth centuries, and several tens of thousands of them survive across Ireland in varying states of preservation. This one at Coolnacrutta is a bivallate example, meaning it has two enclosing elements rather than the single bank more commonly seen. An inner bank, roughly six metres wide at its base and two metres high on its outer face, is separated from a second, lower outer bank by a flat-bottomed, V-shaped fosse, the term for the ditch dug to create the banks in the first place. The entrance, facing east, survives as a gap of about three metres, with a causeway crossing where the fosse has been partially filled in over the years. The positioning on the valley floor, with clear sightlines in every direction across the surrounding land, suggests the enclosure was as much about visibility and the management of livestock as it was about any serious defensive ambition.