The trees press in around it, but this particular area was spared when the plantation went in around 2010, and the reason is that something older is there beneath the undergrowth. The outline of an early fort survives, its edges marked by a low scarp barely a quarter of a metre high and the faint trace of a waterlogged fosse, which is the ditch that would originally have enclosed the site. Neither feature announces itself with any drama; both are intermittently visible at best, more sensed than seen.

The fort sits on a drumlin ridge running roughly north-west to south-east. Drumlins are the low, smoothed hills left across much of Ulster and Connacht by glacial activity, and they were frequently chosen as settlement sites in early medieval Ireland, offering a degree of elevation and visibility without the effort of building on more demanding terrain. A circular enclosure of this kind, defined by an earthen bank and external ditch, is broadly what archaeologists mean by a ringfort, one of the most common monument types in the Irish landscape, with tens of thousands recorded across the island. Most were farmsteads rather than military installations, the word "fort" carrying older and looser connotations than it does today. The Drumderg example is modest in scale and, given its current condition, probably never ranked among the more elaborate examples. Michael J. Moore's Archaeological Inventory of County Leitrim, published in 2003, recorded its essential dimensions and the evidence, slight but legible, of its enclosing features.