It only came to attention through aerial imagery, which revealed what ground-level observation alone had obscured: a grass-covered platform, roughly thirty-two metres north to south and thirty-four metres east to west, sitting on the level top of a low drumlin where the river bends around the eastern side of the hill.

A rath is a ringfort of early medieval date, typically a raised circular enclosure defined by an earthen bank and, often, an external ditch known as a fosse. At Lissananny Beg the defining feature is a scarp, a slope marking the edge of the platform, which survives to between half a metre and three-quarters of a metre in height depending on where you measure it. The eastern arc is the most legible, partly because the natural ground falls away there, lending the scarp an added presence. Elsewhere the break of slope is gradual and indistinct, consistent with partial levelling at some point, probably during agricultural improvement. On the western side a later field bank running north to south has cut across the scarp, interrupting it. A band of damp ground about four metres wide on the southern arc may indicate the line of a fosse, though it cannot be traced around the rest of the circuit. That original ditch, if it existed, has long since been absorbed into the field. A more recent field bank, two metres wide, crosses the entire rath on an east to west axis, accompanied by a silted-up field ditch, and a few hawthorn trees have taken root along its course. The effect is of two separate agricultural systems laid one on top of the other, each indifferent to what came before.