This is a rath, a type of early medieval ringfort constructed from banked earth rather than stone, and the example at Mullenroe is a well-preserved specimen of a form that was once extraordinarily common across Ireland. Thousands of ringforts survive in the Irish landscape, yet each one carries the faint outline of a domestic life organised around enclosure, with the circular bank marking the boundary of a farmstead rather than a military fortification.

The Mullenroe rath measures 39 metres across in both its north-south and east-west axes, making it a near-perfect circle. Its earthen bank still stands to a height of 1.3 metres, and outside that bank runs a fosse, a defensive ditch, still 0.5 metres deep and waterlogged in parts. The entrance to the east is a causeway 2.6 metres wide, its edges defined by stone facing on each side, giving the threshold a deliberately constructed quality. A second gap exists to the south-south-east, though whether this is an original feature or a later break is harder to say. That the fosse retains water speaks to the site's reasonable state of preservation; the underlying topography of the northwest-facing slope evidently still holds moisture in the old cut.