What marks it out is the combination of its setting and its survival: a monument that occupies the highest point of a glacially deposited hill, a drumlin being one of those elongated, egg-shaped mounds left across the Irish midlands and Ulster by retreating ice sheets, and which has held its form well enough that its internal dimensions can still be measured, roughly 95 metres along its longer north-east to south-west axis and 64 metres across.

The monument appears on the 1834 edition of the Ordnance Survey six-inch map, where it is shown as an oval enclosure formed by field banks and labelled in the gothic lettering that the early OS cartographers reserved for antiquities, identifying it simply as a fort. In Irish archaeology, the word fort in this context generally refers to a ringfort, a type of enclosed settlement that was widespread in early medieval Ireland, typically defined by one or more earthen banks and ditches. At Lenagh, the enclosure is defined by a scarp, a low but distinct earthen edge or slope, which varies slightly in width and height around its circuit, from about 2.6 metres wide and just over a metre high on the north-east side to 2 metres wide and nearly 1.5 metres high to the west. There is no visible fosse, the term for the external ditch that commonly accompanies such earthworks, though a broad entrance roughly 8 metres wide survives on the east-south-east side. Outside the scarp, field banks and hedges follow much of the perimeter, in places sitting on the scarp itself rather than beside it, suggesting the old boundary has been incorporated into later field management over the generations.