It measures approximately 31 metres north to south and 28 metres east to west, and were it not for the faint rise of an earthen bank along one arc of its perimeter, it might pass entirely unnoticed. No entrance can now be identified. What survives is a slight scarp for much of the circuit, with the more substantial remnants of an original bank and external fosse, a defensive ditch dug around the outside, absorbed into a later field bank and drain along the eastern to south-western stretch.

The only cartographic record naming the site as a fort comes from the 1834 edition of the Ordnance Survey six-inch map, where it is marked in the distinctive gothic lettering the OS reserved for antiquities. That label suggests the site was already understood locally as something ancient at the time of the first systematic mapping of Ireland. The earthwork itself is a ringfort, or the remains of one, a type of enclosed farmstead common in early medieval Ireland, typically dating from roughly the sixth to the twelfth centuries. Thousands were built across the island, their circular banks offering a degree of security for a family, their livestock, and their stores. At Mullanacross, the choice of position on the ridge crest, with a southward-facing slope, would have offered both visibility over the surrounding terrain and a degree of natural drainage, practical considerations that seem to have guided ringfort builders across the country.

The site sits on a drumlin, one of the countless smooth, elongated hills formed from glacial deposits that give this part of Monaghan its characteristically humped and enclosed quality. That underlying geology shapes everything here, the ridge, the slope, and the way the old earthwork has settled and blurred over centuries into the field system around it.