What had been mapped as a circular enclosure roughly 25 to 30 metres in diameter was recorded, less than eighty years later, as only a fragment. The shrinkage tells a quiet story about how the Irish landscape was worked and reworked across the nineteenth and early twentieth centuries, one earthwork at a time.

A ringfort, or rath, is one of the most common archaeological monument types in Ireland, typically a roughly circular area enclosed by an earthen bank and ditch, thought to have served as a defended farmstead during the early medieval period. The Derrygirraun example sits on a shelf partway down the south-east-facing slope of a drumlin, the low elongated hill formed by glacial deposition that gives this part of Roscommon its gently rolling character. What survives today is a D-shaped patch of grass and scrub, approximately 15 metres north to south and 11.5 metres east to west, which has been left unplanted within an otherwise dense coniferous forest. A low scarp, between 0.2 and 0.4 metres high, still traces the southern, western, and northern edges of the enclosure. The eastern side is gone, cut away by a quarry scarp that rises to 2 metres, which accounts for much of the loss visible between the two map editions. Roughly 190 metres to the north-north-east lies a cashel, a related monument type in which the enclosing boundary is built from stone rather than earth, suggesting this corner of Derrygirraun was once a more densely settled place than its current forestry cover implies.