This is a rath, the most common type of early medieval settlement monument found across Ireland, built by farming families who enclosed their homestead with a raised bank and an outer ditch, known as a fosse. The low but deliberate geometry of the place is what sets it apart from the surrounding landscape; this was not a defensive fortification in the military sense so much as a statement of household territory, a way of organising space and resources on a working farm.

The enclosure at Derrylusk measures roughly 35 metres across and is positioned on a south-east-facing slope of a north-east to south-west drumlin ridge, the kind of glacially deposited oval hill that gives Monaghan its famously humped and intimate topography. The earthen bank that defines the interior is around five metres wide at its base and rises to between two and three metres on the exterior face, making it a substantial piece of earthwork even now. An outer fosse survives best along the north-east to east-south-east arc, where it is over five metres wide at the top and drops to around 1.3 metres in depth. The entrance, a gap roughly two and a half metres wide at the base, faces south-east, and a low causeway, still about 40 centimetres high and over three metres wide, bridges across the fosse to meet it. The south-east orientation of the entrance is typical of many raths across Ireland, often interpreted as a preference for morning light and shelter from prevailing westerly winds.