The site at Mountplummer is a D-shaped earthen fort whose straight southeastern edge is simply the top of a steep natural incline, the drop itself doing the defensive work that elsewhere would require banks and ditches. It is an economical piece of thinking, and easy to miss entirely if you do not know what you are looking for.

The enclosure measures roughly 28.5 metres north to south and 38.2 metres east to west. Two concentric earthen banks curve around from the southwest to the east, set about 3.25 metres apart, with a fosse, an external ditch, running along the outer side from the southwest to the northwest. The fosse is not especially deep, recorded at 0.65 metres, but spans 2.6 metres in width. The inner bank stands to around a metre on its interior face and slightly more on its exterior, while the outer bank reverses that relationship. Earthen ringforts of this general type are among the most common field monuments in Ireland, typically associated with early medieval farming settlements, though the specific history of occupation at Mountplummer is not documented in the survey record compiled by Denis Power, uploaded in August 2011. What the record does note is that the earthworks are substantially worn where cattle have repeatedly crossed them at the northwest and northeast, creating breaks in both banks that are now the practical entry points into the interior.

The interior slopes gently down toward the southeast and is today covered by mature deciduous trees, which gives the space an enclosed, dimly lit quality quite different from the open pasture surrounding it. Dense vegetation masks much of the enclosing earthwork from the outside, so the banks can be difficult to read clearly until you are already among them. The best-preserved stretch runs from the southwest around to the west-northwest, where the banks have escaped the worst of the cattle damage. Because the site sits in working pasture, visiting is a matter of navigating the ordinary courtesies of Irish farmland. The trees and undergrowth mean that late autumn or winter, when the leaf cover is down, offers the clearest view of what remains of the earthworks.