A drumlin, for those unfamiliar with the glaciated landscapes of the west of Ireland, is a smooth, elongated hill shaped by retreating ice, and their southern flanks were frequently chosen for early enclosures, offering drainage and a degree of natural shelter. What survives at Castlebin is a subcircular enclosure measuring roughly 38.5 metres north to south and 31 metres east to west, defined not by a standing bank or wall but by a scarp, a subtle change in ground level that traces the circuit of what once stood here.

The monument has not fared well over the centuries. Field boundaries slice across it at several points, suggesting that at some stage the enclosure was simply absorbed into the working agricultural landscape around it, its outline subordinated to the more immediate logic of land division. More significantly, the south-western to western arc has been quarried away, removing a portion of the circuit entirely. Enclosures of this general type are among the most common archaeological features in the Irish countryside, ranging in date from the Bronze Age through to the early medieval period, when they typically functioned as ringforts, the farmsteads of farmers and petty lords. Without excavation, though, assigning a precise date or function to this particular example is not possible, and what remains is better understood as a shape in the land than as a legible monument.