Half a ringfort, more or less, the western arc of an enclosure that once measured roughly 17 metres across, its eastern half long since levelled or absorbed into the surrounding landscape. What remains is enough to read, just about, if you know what you are looking at.

Ringforts, known in Irish as raths when defined primarily by an earthen bank and fosse, were the most common form of rural settlement in early medieval Ireland, typically dating from around the sixth to the twelfth century. They served as enclosed farmsteads, the bank and surrounding ditch offering both a degree of security and a clear boundary for livestock. At Trienearagh, the surviving portion runs from west through north to east, forming a gentle semi-circular arc. The bank is low, rising only around 0.6 metres above the interior, but notably wide at the base, between 5 and 10.6 metres across in places. The fosse, a shallow exterior ditch that would originally have reinforced the bank's defensive or enclosing function, is traceable along the same arc; it reaches about 2 metres in width and 0.6 metres in depth where it is clearest, to the west and north-northeast. These are modest dimensions, and the site makes no dramatic impression on the ground.