The raised platform, roughly 29 metres across, is what remains of a rath, the Irish term for a ringfort, which was the most common form of farmstead in early medieval Ireland. These enclosures, typically built between around 500 and 1000 AD, sheltered a family and their livestock within a bank and ditch arrangement that served as much as a marker of status as a defensive boundary.

At Kilsellagh, the enclosure survives in partial form, and reading what remains requires a little patience. On the northern and north-western arc, the enclosing bank has been removed entirely, and two later field boundary banks have since been built directly across the gap, one running north-east to south-west, the other east to west. These boundaries have absorbed and erased part of the original circuit, a common fate for ringforts that sat in agriculturally useful ground. Where the bank does survive, it is broad rather than tall, measuring just over three metres wide with an internal height of only 0.3 metres. Outside it runs a fosse, a rock-cut or earthen ditch, here about five metres wide and again relatively shallow. On the south-eastern to west-north-western arc, where the bank disappears, the edge of the enclosure is instead defined by a natural scarp, a steep drop in the ground surface of about 1.4 metres, which the original builders appear to have incorporated deliberately into their design. The natural slope of the land, falling away sharply from the site on the south-east to north-west axis, would have made this section of the perimeter naturally defensible without any additional earthwork. No trace of the original entrance survives in a recognisable form.