The earthwork itself is circular, roughly 23 metres across, and what defines it is a scarp, essentially a steep-sided earth bank cut and piled to form a wall-like boundary, here standing about two metres high with a slight internal lip running along its inner face. To the west there is also an external fosse, a shallow defensive ditch roughly half a metre deep, while a narrow ledge survives at the base of the scarp on the northern side. The entrance, a gap six metres wide, opens to the east.

Ringforts, known in Irish as raths when built primarily from earthworks rather than stone, are among the most common archaeological monuments in the Irish countryside, with many thousands recorded across the island. Most date broadly to the early medieval period, roughly the fifth to twelfth centuries, and typically served as enclosed farmsteads for a single family or small community. The enclosing bank and ditch were less about military fortification and more about marking territory, controlling livestock, and providing a degree of security. What makes the Dunmarklun example quietly interesting is the possible presence of a souterrain beneath or near the interior. A souterrain is an underground stone-lined passage or chamber, often built in association with ringforts and used variously for storage, refuge, or both. The interior and the scarp are now heavily overgrown with ferns, which lends the site a closed, half-buried quality, as though the ground itself has slowly resumed custody of what was once a working domestic space.