Before the reservoir was constructed, archaeologists had a rare opportunity to examine this small enclosure on a west-facing slope above what had been marshy ground, and what they found was a site of quiet structural complexity, now gone from view entirely.

The fort was bivallate, meaning it was defined by two concentric circuits of earthworks rather than the single bank and ditch more commonly encountered. The interior measured 23 metres across, with the full extent of the outer earthworks reaching 41.5 metres in diameter. An earthen inner bank enclosed a fosse, a defensive ditch roughly a metre deep and more than two metres wide, which was itself separated from the outer bank by a flat shelf of ground called a berm. Entrances into the interior were arranged carefully: narrow gaps in the inner bank at the north-east and west-south-west, and a more substantial opening at the south-south-east, positioned opposite a stone causeway across the ditch. A second causeway of large stones at the north-east may have been added later. More unusual still was an L-shaped bank projecting outward from the inner circuit at the south, crossing the fosse and then turning east to meet the main entrance, a feature that would have controlled or funnelled movement into the enclosure. The excavation, carried out by Macalister in 1943, revealed relatively little in the way of internal features; only an arc of irregular paving in the northern part of the interior survived to be recorded. Ringforts of this kind were the farmsteads of early medieval Ireland, typically occupied between roughly the fifth and twelfth centuries, and the double-bank arrangement here would generally have indicated a household of some standing in the local community.