The effort required was not trivial: on the southern side, the ground was cut back into the slope; on the northern side, the excavated material was piled up to compensate, producing a roughly level interior platform approximately 35 metres across. The result, from a distance, is easy to miss entirely.

This is a rath, the most common type of early medieval settlement monument in Ireland. A rath typically consists of a roughly circular area enclosed by one or more earthen banks with an external fosse, the fosse being a ditch that both provided the raw material for the bank and added to its defensive or symbolic height. In their heyday, from roughly the sixth to the twelfth centuries, raths were the farmsteads of free farming families across the Irish countryside; tens of thousands were built, and many thousands survive in varying states of preservation. The Glenduff example sits on a north-facing slope and retains its bank, though just barely: the interior height of the bank now measures only around 0.2 metres, while the exterior face rises to about 0.9 metres. Cattle grazing in the enclosure have worn the earthwork down considerably, and breaks have opened in the bank to the north and west. The external fosse, once a defining feature, survives only as a faint depression in the turf. What remains is enough to read the original form, but only if you know what you are looking at.