The slab measures roughly 2.8 by 2.94 metres, which is to say it is very nearly the footprint of a modest room, and it rests at an angle: supported at its south-western end by a triangular upright stone, it slopes downward to the north-east where it meets the ground. That tilt, combined with a scatter of rubble both within and around the structure, and a separate mound of stones 1.5 metres to the north, suggests what was once a more complete monument has gradually slumped and shed its components over a very long time.

Portal tombs are among the oldest surviving built structures in Ireland, generally dated to the Neolithic period and characterised by two tall upright portal stones at one end supporting a large capstone, often dramatically tilted. The Lissava example was identified as a possible portal tomb by Paul Walsh in 1991, though the tentative classification reflects how much of the structure has been lost or obscured. Assuming the triangular stone is a side-stone rather than one of the portals, the tomb would have been aligned roughly north-west to south-east. A curious detail is the linear depression running across the centre of the capstone, between 23 and 32 centimetres wide and only a few centimetres deep, whose origin is unclear. Equally notable is a loose rectangular block of quartz lying immediately north-east of the tomb; quartz was frequently placed at or near megalithic monuments in prehistoric Ireland, apparently with deliberate intent, though its significance at this site is unknown. The tomb sits on a break in the slope with extensive open views to the east and south, a siting that is typical of portal tombs elsewhere in the country, many of which seem to have been positioned with an eye to the wider landscape.