The cairn, roughly circular and about 22 metres in diameter, has sunk into the bog over millennia, its defining boulders now only reliably visible along the southern and eastern edges. At the centre, three upright stones, known as orthostats, enclose a chamber barely a metre square, with a capstone that has shifted slightly from its original position. It is, in the language of archaeology, much denuded, which is a polite way of saying that time and the mountain have had their way with it.

Passage tombs of this type were built during the Neolithic period, generally between around 4000 and 3000 BC, as collective burial monuments. They typically consist of a stone-lined passage leading to a burial chamber, all covered by a mound of earth and rubble. At Ballinascorney Upper, the passage appears to have run north to south, and the three surviving orthostats, standing between 0.49 and 0.7 metres in height, still loosely define the square chamber at its heart. Many of the kerbstones that would once have formed a neat boundary around the cairn have been displaced, making the monument's original extent harder to read on the ground. The site is documented in Michael Herity's 1974 survey of Irish passage graves, and more recently in work by Redmond and Mac Aonghusa published in 1994.

The tomb lies northeast of Seahan's summit, and reaching it involves crossing open upland terrain where the ground can be soft and uneven, particularly after rain. From the monument's position, the Liffey valley spreads out to the north, and on a clear day the passage tombs at Mountpelier on the adjacent ridge come into view, a reminder that this was a landscape of deliberate, repeated Neolithic activity rather than a single isolated burial site. Underfoot, the peat covering much of the cairn can make individual stones difficult to distinguish from the surrounding ground, so moving slowly around the southern and eastern arcs, where the boulders are best preserved, gives the clearest sense of the original structure.