An esker is a long, winding ridge of gravel and sand deposited by meltwater streams flowing beneath retreating glaciers at the end of the last Ice Age, and they tend to run through the Irish midlands and west like raised causeways above the surrounding bogland. Someone, at some point in the early medieval period, chose this particular east-west ridge in County Mayo as the place to build their enclosure, and the choice was plainly deliberate: from the top of the esker, the Ox Mountains, Nephin, and Croagh Patrick are all visible on the horizon.

A rath, or ringfort, is an enclosed settlement of the early medieval period, roughly between the fifth and twelfth centuries, typically defined by one or more earthen banks and used as a farmstead or residence. This one measures roughly 25 metres north to south and 33 metres east to west, and it straddles the full width of the esker ridge so that its defining scarp, the steep outer edge of the earthen platform, lines up neatly with the natural break of slope on both the north and south sides of the ridge. The builders made the most of the natural topography rather than working against it. At the north-west to north-east arc the scarp has been deliberately built up to about 1.4 metres to create a level interior, compensating for the natural slope of the ridge. Elsewhere the scarp is considerably lower, and a notably shallow section on the western side may mark where the original entrance once was. To the south, the esker drops sharply to damp, low-lying pasture, where a second rath stands just 200 metres away. To the north, the slope is more gradual, falling away to a broad expanse of bog that stretches for over a kilometre to the banks of the River Moy.