Nobody knows where the entrance once was.

The site at Glasdrummond is a ringfort, the most common type of early medieval enclosure in Ireland, typically used as a farmstead or place of defence between roughly the fifth and twelfth centuries. What makes this example quietly interesting is the layering of its boundary. The outer edge is marked by a scarp, a low earthen slope, which on the southern side still retains traces of stone facing, suggesting something more deliberately constructed than a simple earthen bank. Beyond that runs a fosse, a ditch cut into the ground, measuring around three metres wide at the top and dropping to about seventy centimetres in depth. The whole enclosure measures just over thirty-nine metres across at its widest, placing it within the typical range for a ringfort of this kind. It sits on top of a drumlin, one of the smooth, elongated hills formed by glacial deposits that give County Monaghan much of its rolling, hummocky character, and the position would have offered clear sightlines across the surrounding landscape. The ground slopes away slightly to the northeast, and a hedge now runs along part of the perimeter, the living successor to whatever boundary material once stood there.