The rath, as this type of earthwork is known, is a ringfort, one of roughly 45,000 surviving examples scattered across Ireland, most of them dating to the early medieval period. They were typically the enclosed farmsteads of prosperous families, defined by a raised bank and a surrounding ditch rather than stone walls. The one at Formoyle sits on a gentle north-north-westerly slope of wet pasture, and its dimensions have been carefully recorded: a circular area of about thirty metres across, enclosed by an earthen bank nearly five and a half metres wide, with a fosse, or external ditch, running along its outer edge.

What makes this particular example quietly interesting is the layering of time visible on its surface. A later field boundary, built from earth and stone and running roughly north-north-east to south-south-west, has been constructed directly on top of the western portion of the original bank. This is a common enough fate for ancient earthworks in agricultural Ireland, where later generations found a ready-made ridge of ground useful for dividing fields and thought little about what lay beneath. The original entrance to the enclosure has been entirely lost, possibly consumed by that same later activity, possibly simply eroded away across the intervening centuries. The bank itself survives to an internal height of only about forty centimetres, and the fosse to a depth of roughly twenty centimetres, so neither is dramatic to the eye. The site commands good views across the surrounding landscape in all directions, which likely explains why the location was chosen in the first place.