It is recorded as a rath, the Irish term for a ringfort, which was typically a circular or oval enclosure of raised earth used as a farmstead during the early medieval period. But local tradition holds that it also served, at some point, as a children's burial ground, a cillín, of the kind used across Ireland for unbaptised infants and others excluded from consecrated ground. No graves are visible inside the enclosure today, yet a low mound of stones, roughly four to five metres across, sits just to the south-west beside an ash tree, and may mark where those burials lie.

The rath itself is a modest but clearly defined feature: an oval raised platform measuring about 19.5 metres north to south and 11.2 metres east to west, its edges formed by a sod-covered stony scarp that rises to 1.4 metres on the north and north-west sides, dropping to around 0.8 metres at the east and south. A short stretch of the scarp on the south-west has been quarried away at some point, leaving a gap in what would otherwise be a continuous circuit. The site sits on a low east-west ridge in pasture land, close to the Roscommon county boundary, and commands a wide northward view: Knocknashee to the north, Knocknarea to the north-north-east, and the long flat profile of Ben Bulben further to the north-east across the county border into Sligo.

The combination of a probable early medieval enclosure and a tradition of marginal burial in the same small space is not unusual in the Irish landscape, where the two uses overlapped for centuries, sometimes because raths were already understood as liminal or otherworldly places, set apart from ordinary ground. What makes Derrynabrock worth pausing over is the uncertainty that remains: the stones by the ash tree may be graves, or they may not be, and the interior has yielded nothing visible to confirm the tradition one way or the other.