The structure is classified as a portal tomb, a type of megalithic monument typically characterised by two tall upright stones framing the entrance and a large capstone tilted dramatically overhead, though in this case the capstone is gone. What remains is the chamber itself: five orthostats, or large upright slabs, arranged to form a rectangular space roughly three and a half metres long and approaching two metres wide, aligned roughly north-northwest to south-southeast. Two tall, well-matched portal stones mark the southern entrance, set just inside the line of the flanking sidestones in the manner typical of the form.

Portal tombs are among the earliest monumental structures in Ireland, generally attributed to Neolithic communities farming and settling the landscape from around 4000 BCE onward. They functioned as communal burial places, though their siting often suggests other purposes too, whether territorial, ceremonial, or cosmological. The Leagun tomb adds a layer of ambiguity to this already complex picture: archaeologists have noted that it could equally be interpreted as the rear chamber of a court tomb, a related but distinct monument type in which a forecourt open to the sky leads into one or more covered galleries. The distinction matters because the two forms imply somewhat different ritual uses and social contexts, and this tomb sits awkwardly between them. Traces of a covering mound survive to the south and west, which would once have enclosed much of the structure, and the rear of the chamber is tucked close to the natural rock outcrop, a detail that may have been as deliberate a choice for Neolithic builders as the view south over the bay.