What remains is the surviving portion of a ringfort, one of the most common monument types in Ireland, a circular or near-circular enclosure defined by an earthen bank and external ditch, typically dating to the early medieval period and associated with farmstead settlement. At Tonyglassan, the bank survives most clearly at the southern arc, where its base measures nearly five metres across and rises to an external height of over two metres, with a fosse, the accompanying ditch, running along the western to north-north-western side. A narrow entrance gap, just over two metres wide, opens at the south-south-east.

The site is subcircular in plan, measuring roughly 35 metres north-west to south-east and just under 30 metres in the other direction. By 1995, however, recorded survey work found that the interior had been cleared and a significant portion of the perimeter, from the north-west around through east to south-west, had been removed. What that clearance involved, whether agricultural improvement, drainage work, or some other intervention, is not specified, but its effect was to leave only a fragment of what was once a complete enclosure. The southern and south-eastern arc, with its legible bank profile and entrance, now represents the most intact section of a monument that elsewhere has been considerably reduced.