Aerial photographs reveal it as a roughly circular feature about thirty metres across, but standing on the hillside you encounter something more ambiguous: a low, subrectangular platform, grass-covered, its top measuring approximately twelve metres east to west and nine metres north to south, widening at its base to around twenty-two by twenty-one metres. The height varies noticeably, rising from a modest thirty-five to forty-five centimetres on the eastern and southern sides to between one and two metres on the western and northern faces, where the slope gives the structure more presence. Between this platform and the faint curve of a low bank to the east and south lies a berm, a flat strip of ground separating two earthwork elements, here running seventeen to twenty metres wide.

Earthworks of this kind rarely announce their purpose clearly, and this one is no exception. What can be said is that it does not stand in isolation. Approximately a hundred and sixty metres to the east-south-east lies a rath, the remains of a roughly circular enclosure of a type common across early medieval Ireland, typically formed by a raised bank and ditch and used as a farmstead or small settlement. Whether the platform on Ardmullan Hill was related to that enclosure in function or in period is not recorded, but their proximity on the same hillside suggests they may share some part of the same landscape history. The position near the top of a north-facing slope, visible from the air as a clean circular outline, points to deliberate construction rather than natural accident, though what that construction served remains an open question.