From a distance it looks like little more than a gentle swelling in the field, but the geometry gives it away: a circular earthwork roughly 24 metres across, its enclosing bank still standing nearly a metre high on the interior side and more than three metres wide at its base. Outside the bank runs a fosse, a shallow ditch cut to reinforce the sense of enclosure, though it has disappeared along the south-west to north-east arc, absorbed over centuries by the ordinary business of farming.

The site belongs to a class of monument known as a rath or ringfort, the most common archaeological feature in the Irish landscape. These circular enclosures, built primarily during the early medieval period, roughly the fifth to twelfth centuries, served as farmsteads for a single family or small kin group. The bank and fosse defined a boundary as much social and legal as defensive, marking the household off from the wider world and providing some protection for livestock. At Formoyle, the survival is partial but legible. A field boundary bank now runs along the south-west to north-west portion of the fosse line, and a lane follows the outer edge of the missing north-west to north-east section, suggesting that later agricultural arrangements quietly absorbed the monument's footprint without entirely erasing it. The original entrance has not been identified; what gaps do exist in the bank are narrow livestock tracks, the widest of them less than a metre across, worn through at the north-north-east, south-east, and south-south-west.