It is the intimacy of the arithmetic: a single stone chamber measuring roughly 2.8 metres in length, covered by one large trapezoidal roofstone prised directly from the surrounding pavement, and yet that small space held the commingled remains of at least 35 people. A portal tomb is a megalithic burial structure in which two tall upright stones frame a narrow entrance, supporting a capstone that typically tilts upward at the front to create a distinctive elevated profile. Poulnabrone sits on bare limestone pavement in the high Burren, visible from the main north-south routeway through the region and about 7.5 kilometres south of Ballyvaughan, one of some 184 portal tombs known in Ireland, most of them concentrated in the northern third of the country and in south Leinster.

Excavations carried out between 1986 and 1988 as part of a conservation project revealed that the upright stones, or orthostats, rest directly on the limestone beneath, with the great roofstone helping to pin them in place and a low surrounding cairn of limestone slabs providing lateral support. The bones recovered from the chamber were unburnt and thoroughly mixed, suggesting that bodies were interred successively over many generations and that earlier remains were moved aside, manipulated, or partially removed to make room. The dates run from approximately 3800 to 3200 cal BC, placing the earliest burials at the very opening of the Irish Neolithic period. Men, women, and children of all ages are represented. Isotopic analysis of the remains pointed to a largely terrestrial diet with little animal protein, and with one exception every individual appears to have grown up on the carboniferous limestone of the Burren itself. Alongside the human bones, excavators found cattle, sheep, goat, and pig remains, along with 42 artefacts including a polished stone axehead, chert and flint tools, two stone beads, and a triangular bone or antler pendant pierced along its centre. The pottery, mostly in the Western Neolithic Tradition, was highly fragmented. A separate and later deposit was found in the small cist-like portico at the entrance: a foetus dating to the middle Bronze Age, accompanied by Bronze Age pottery sherds, a reminder that the site continued to carry meaning long after its original use ended.

The eastern portal stone visible today is a replacement; the original had developed significant horizontal and vertical cracks by the time of excavation and was substituted during the conservation work. A subsidiary roofstone, a large flat slab resting partly on the cairn and partly over the backstone, is visible to the south of the main capstone and is easy to overlook until you read the structure carefully. The tomb stands in open country with no obstruction, and the low oval cairn surrounding it, roughly 10.5 metres north to south, is still clearly legible in the ground.