The raised bank, measuring around 26 metres north to south and nearly 28 metres east to west, has been pressed into service as one wall of a narrow rectangular field. The eastern and western arcs of the enclosure align with straight fence lines on either side, and the western arc carries a post and wire fence along its crest. In places the bank has been faced with drystone walling to sharpen its profile, giving it an almost vertical external slope that looks more like a field boundary than an ancient monument, which is precisely what it has become to the farmers who work around it.

The structure is a rath, the Irish term for a ringfort, a type of enclosed farmstead built across Ireland roughly between the early medieval period and the Viking Age, typically from around the fifth to the twelfth century. Thousands survive in varying states across the country, usually as earthen banks enclosing a circular area where a family and their animals would have lived. At Treanfohanaun, the interior remains level and is covered in long, hummocky grass, and the ridge-top position would have given its original occupants clear sightlines in every direction, a characteristic that appears to have been a deliberate choice rather than coincidence. Two narrow breaks in the bank, each less than two thirds of a metre wide, cut through at the north-north-east and at the south, allowing passage through the enclosure from one part of the field to the other, but these are almost certainly later agricultural modifications. The more likely original entrance lies at the south-east, where a short stretch of drystone walling now blocks what appears to be an earlier gap in the bank.