The enclosure at Mullaghboy is roughly thirty metres across, and despite being heavily overgrown it preserves a surprisingly legible set of defences: an inner bank, a flat-bottomed fosse (a defensive ditch, dug to slow or stop an approach), and a further outer bank beyond that. Together they form concentric rings of earth that would once have made the interior a reasonably well-protected space.

The earthwork is most complete along its upslope, northern arc, where the outer bank still rises about 1.6 metres above the fosse floor and the ditch itself stretches nearly seven metres across at the top. On the southern, downslope side, erosion and time have done their work: the inner bank has slumped to a low scarp and only faint traces of the fosse remain. This asymmetry is fairly common in ringfort-type enclosures, where the natural advantage of the slope reduces the need for elaborate defences on the downhill face. Ringforts, the most numerous class of monument in the Irish countryside, were typically built during the early medieval period, roughly 500 to 1000 AD, and functioned as enclosed farmsteads rather than purely military fortifications. Whether Mullaghboy fits neatly into that category or represents something slightly different is an open question; the original entrance has not been identified, which makes interpretation harder than usual.