It is an odd collision of the ancient and the agricultural, the kind of thing that happens when a landscape is farmed continuously for over a thousand years without quite erasing what came before.

A rath, or ringfort, is an enclosed settlement typical of early medieval Ireland, usually circular in plan and defined by one or more earthen banks with a corresponding ditch, known as a fosse, dug to provide the material for those banks. The example at Culleen Beg sits on its hillock amid rough, gently undulating pasture and was already recorded as a circular earthwork on the Ordnance Survey six-inch map of 1837. When the monument was described in detail in 1970, surveyors found a roughly circular enclosure approximately 36 metres in diameter. The inner bank remains high and steep, and the fosse, which is nearly four metres wide at its top and has a notably flat base, is still well defined. A gap on the eastern side, around seven metres wide at the top, is thought to mark the original entrance, though no causeway survives to confirm this. Beyond the inner bank, faint traces of an outer bank survive, barely distinguishable from the natural slope of the hillock. At the northern side, a partially quarried semi-circular annexe, a secondary enclosure added to the main rath, adjoins the outer bank, though quarrying has removed much of it. The fosse and outer bank have also been cut away on either side of the eastern gap.

What is particularly legible about this site is its persistence. Aerial photography shows it as a tree-planted earthwork, the ring of vegetation tracing the line of the banks even where the earthworks themselves have been reduced or built upon. The hay barn in the interior is a reminder that working farms and ancient monuments in Ireland frequently share the same ground, each making what use it can of a convenient rise in a Westmeath field.