The earthen bank that once enclosed this space is worn low now, visible only along its western and northern arcs, while a wide external fosse, the ditch dug to raise that bank in the first place, survives more clearly from the north around to the east-southeast. The entrance, a narrow gap just 1.2 metres wide, opens to the north, and at the centre of the interior a rectangular raised platform suggests the outline of a hut site, the footprint of a dwelling set inside a defended farmstead.

This is a rath, the most common monument type in the Irish landscape. Raths are ringforts built of earth rather than stone, constructed mostly during the early medieval period, roughly between the fifth and twelfth centuries, as the enclosed farmsteads of free farming families. They vary considerably in size and elaboration, but the Ballynacor example, measuring approximately thirty metres north to south and twenty-nine metres east to west, sits within the ordinary range. What gives it particular character is its position on an east-southeast facing slope with open views to the south, the kind of siting that would have made practical sense for anyone managing livestock and watching the approaches to their land. The monument has not escaped the ordinary pressures of agricultural life. A twentieth-century field fence cuts across it on a roughly east-west line, and the bank south of that fence has been reduced to little more than a low scarp in the ground, a faint edge rather than a standing feature.