The site is a rath, the most common type of early medieval settlement monument in Ireland, typically consisting of a raised earthen bank defining a roughly circular enclosure within which a family would have kept livestock and sheltered in timber or wattle structures. This one at Derreen is subcircular rather than perfectly round, measuring roughly 22.5 metres north to south and 25.75 metres east to west internally, with the ridge position giving unobstructed views in every direction, which was very likely the point.

The bank itself is steep-sided, with an outer facing of stone revetment still visible along the south-east arc, and faint traces of an external fosse, a defensive ditch, can be made out at the same quarter. A fosse would originally have deepened the impression of the bank rising above it, making the enclosure feel more substantial than the modest surviving earthwork now suggests. At the east, a gap just over a metre wide is flanked on its south side by a large flagstone, and this may well be the original entrance, the threshold through which people and animals passed in and out of daily life. Inside, the ground is flat, and a discrete patch of overgrown rubble in the northern quadrant, around seven metres long and up to three metres wide, could represent the collapsed remains of an internal structure. The site was recorded on Ordnance Survey six-inch maps in both 1840 and 1916, marked with hachures in the cartographic convention used to indicate earthworks.

A modern stone wall runs across the monument from south to north, and a quantity of field clearance rubble has been piled along its northern perimeter, the familiar fate of an earthwork that has spent generations being treated as convenient storage for stones turned up by ploughing. The ridge-top setting is still legible, though, and the entrance flagstone, if you find it, rewards a pause.