The enclosure at Derreenacrinnig measures nineteen metres across, its perimeter wall built in a method that speaks to considerable care: an inner and outer row of upright contiguous slabs with an infill of smaller stones and earth between them, producing a wall still standing to around 0.6 metres in height and over a metre thick in places. The southern entrance, defined by two parallel standing slabs of noticeably different heights, is among the better-preserved sections, along with the arc running southeast to west. The northeast portion has suffered the most, reduced to a more disordered arrangement of stone. From inside the enclosure, the ground is slightly uneven underfoot, and views open to the southwest towards Castledonovan.

Enclosures of this kind, sometimes called ring forts or raths depending on their construction, were a common feature of early medieval Irish settlement, typically serving as farmsteads or places of habitation enclosed for the protection of people and livestock. The presence of a hut site within the northern quadrant of the interior suggests the space was indeed lived in at some point, while a second hut site sits just outside the enclosing wall to the south-southwest, hinting at a small cluster of activity around this spot. The double-slab entrance at the south is a detail worth pausing over: the two jamb stones differ in height by roughly a third of a metre, giving the threshold an asymmetry that feels oddly human in scale. The valley setting, with its boggy soil and sheltered position, would have offered both grazing and a degree of natural protection to whoever made their home here.