The capstone, a wedge-shaped slab of granite measuring over five metres long and four and a half metres wide, tilts forward at a pronounced angle, its front edge bearing down on two upright portal stones and its weight distributed across three sidestones. On its upper surface, two deep depressions have been worn or worked into the granite, with grooves running from them towards the corner of the stone. Scholars have suggested these may be original drainage features, designed to channel rainwater away from the chamber below, which would make them among the more practical details to survive from the Neolithic period.

A portal tomb, sometimes called a dolmen, is one of the earliest forms of megalithic monument found in Ireland, typically consisting of a roofed chamber defined by tall upright stones at the entrance and a large capstone overhead. The Brenanstown example is particularly well-preserved. Inside, the internal chamber measures just over three metres in length and around one and a half metres wide, separated from a smaller antechamber by a doorstone, with the whole structure oriented to face west. Granite boulders at the rear of the tomb wall may represent the remains of an additional chamber, a possibility noted by Borlase as far back as 1897 and later discussed by Ó Nualláin and Turner in publications from 1983. The monument is recorded as National Monument 291 and has been taken into State Care under Guardianship, which offers it a degree of formal protection unusual for a site in such a sheltered, semi-enclosed location.

The tomb sits within a glen that keeps it partially screened from the surrounding suburban landscape of south County Dublin, which adds to the slight sense of displacement when you encounter it. The valley floor setting is atypical; many portal tombs occupy more exposed positions, and the proximity to the stream and the enclosing banks of the glen give this one a different atmosphere. The roofstone is best appreciated from a low angle, where the full mass of it becomes apparent resting on portal stones standing at 2.6 metres and 2 metres respectively. The drainage grooves on the upper surface require a closer look and are easy to overlook on a first visit.