Eskers are the long, winding ridges of gravel and sand left behind by meltwater rivers that ran beneath the retreating ice sheets at the end of the last glacial period, and they were used as routeways and elevated sites throughout Irish prehistory and the early medieval period. This one carries a building whose date no one has been able to fix with any certainty, which in itself is something worth pausing over.

The site at Rathcosgry is reasonably well preserved. A circular house platform, roughly 8.5 metres in diameter, is outlined by a grassed-over bank that has been revetted on its inner face with large boulders, a construction technique that helped stabilise the bank against collapse. On its south-eastern side, an entrance passageway extends outward for about 5.3 metres and is just under 3 metres wide, marked by two parallel lines of set boulders, enough to give a clear sense of how people once approached and entered the space. Encircling much of the monument from north around through east to south runs a wide, irregular fosse, essentially a ditch or depression in the ground. The irregularity of this fosse is notable; rather than a deliberately shaped defensive feature, it may be the result of quarrying, the boulders used to construct the building and revetment likely having been lifted from the surrounding ground, leaving the uneven hollow that remains today. McCaffrey noted the site in 1952, but beyond that record its chronology remains open.