This is a rath, the most common type of early medieval settlement monument in Ireland, typically dating from roughly the fifth to the twelfth centuries. Built as an enclosed farmstead, a rath consisted of a raised interior area ringed by one or more earthen banks and ditches, providing both a degree of defence and a marker of status for the family who lived within.

The Barnacoghil example is a reasonably well-preserved specimen of this type, measuring around 32 metres in diameter. Its enclosure is formed by a combination of features: an earthen bank along the southern and western arc, between 3.7 metres wide and standing around 1.45 metres on its outer face, and a low scarp running along the northern side that has been worn to little more than a gentle undulation. A fosse, which is simply a defensive ditch, runs along the outer foot of the bank, and beyond that lies a secondary outer bank. A disused trackway and a field fence follow the curve of the monument closely on the south-to-north-northwest side, suggesting the rath shaped the local landscape long after it ceased to function as a settlement. Most intriguingly, within the western half of the interior lies a collapsed souterrain, an underground stone-lined passage or chamber that would originally have served as a place of storage or refuge, and occasionally concealment, for the people who once occupied the enclosure above it.

The ridge setting is itself significant. Raths were frequently positioned on elevated ground, giving their occupants a clear view of the surrounding land. Here the east-west orientation of the ridge would have made the site conspicuous in the local topography, while the layered earthworks, even in their current reduced state, give a real sense of the effort that went into construction.