What makes it worth a second look is its doubling: this is not a simple ringfort, but a bivallate enclosure, meaning it has two earthen banks and two ditches arranged concentrically, with a narrow strip of flat ground, called a berm, separating the inner from the outer works.

The enclosure is almost perfectly circular, measuring around 41 metres across on its longer axis. A causeway across the inner flat-bottomed fosse, or ditch, at the east-north-east provides the entrance, and traces of a second causeway survive at the same orientation across the outer fosse. The inner bank has a base width of roughly 2.3 metres; the outer is slightly broader at 2.6 metres. The causeway itself stands about a metre high at its top. Ringforts of this type, earthen enclosures thought to have served as defended farmsteads, are common across Ireland, but bivallate examples represent a smaller subset, sometimes interpreted as the homesteads of higher-status households in the early medieval period, though the exact chronology of any individual site is difficult to establish without excavation. The Mullan example sits in that ambiguous territory: well-preserved enough in its basic geometry to be read clearly, but without excavation evidence to say who built it or when.