The capstone alone measures nearly four metres in length and three metres wide, and at almost a metre thick it represents a considerable feat of prehistoric engineering. Beneath it, a single displaced orthostat, most likely a former sidestone, is all that remains upright. Dumped field debris surrounds the monument, giving the site a quietly neglected air that makes it easy to overlook what is actually there.

Portal tombs, sometimes called dolmens, are among the most visually distinctive megalithic structures in Ireland, typically consisting of two or more large upright stones supporting an oversized capstone, the whole arrangement once covered by an earthen or stone cairn. What makes the Killonerry example particularly interesting is a note recorded around 1850 by a local observer named Fogarty, who described the structure as it then stood, raised on four pillars roughly three feet above the ground. That description suggests the tomb was significantly more intact in the mid-nineteenth century than it is today, and that the collapse and displacement of its supporting stones has occurred within relatively recent memory, at least in archaeological terms. The site appears in a number of older county and antiquarian records, including references by Mason in 1814, Carrigan in 1905, and Borlase in 1897, indicating it was recognised by scholars of the period even if it never attracted sustained attention.