It is a rath, the most common type of ringfort found across Ireland, and ringforts were the enclosed farmsteads of early medieval Ireland, typically dating from roughly the fifth to the twelfth century. Most consisted of a raised interior platform encircled by one or more earthen banks and ditches, and were home to a single family and their livestock. The one at Greenmount is modest by any measure, yet it preserves enough of its original structure to reward a careful look.

The enclosure is sub-oval in plan, measuring approximately 38 metres north to south and 30 metres east to west. It is defined by an internal bank, a fosse (the ditch or trench dug to create the bank material), and, in places, a second outer bank beyond that. The internal bank is fairly uniform in profile, around 3.45 metres wide, though it has been cut through on the east and west sides by a field drain installed at some point in the agricultural life of the land. The fosse, only about 0.6 metres wide where it survives, is visible from the south-east around to the north-east, before being similarly interrupted. The external bank, wider at nearly 4.7 metres, can only be traced along the southern arc, from south-east to south-west. Inside, the ground is noticeably uneven, and the most plausible explanation is quarrying activity, concentrated in the southern and south-eastern portions of the interior. There is also a small circular mound near the centre of the enclosure, roughly 9 metres in diameter and only about 0.1 metres high, whose purpose is unrecorded.

The rath sits on a ridge with open views in all directions, which would have made good sense to whoever chose the location. Today the setting is gently rolling farmland, and the earthworks are low enough that they read more clearly from the edge of the field than from any distance. The damage from the field drain is worth noting before a visit; the western and eastern sections of the bank and fosse are the least intact, while the southern arc preserves the most legible sequence of bank, ditch, and outer bank. The small interior mound is subtle and easy to overlook underfoot.