When Dowth Hall was built around 1744 to 1765 for the fifth and sixth Viscounts Netterville, the tomb beneath it had probably already been dismantled, its stones absorbed into the landscape long before the first foundations were dug. It was not until archaeological monitoring by Clíodhna Ní Lionáin, carried out from August 2017 to mid-February 2018, that the scale of what lay beneath the Hall's south and west sides began to emerge.

The cairn uncovered during excavation was substantial, somewhere between 38 and 53 metres in diameter, and composed of angular stones laid over a prepared surface of introduced sods. Along its southern edge, six greywacke kerbstones survived, three of them still in their original positions. One of these, kerbstone K2, has a surface almost entirely covered with concentric circles and spirals, and two others carry faint carved motifs. The straightness of the kerb line, combined with a capping of quartz stones, suggests an entrance passage was once nearby, though no direct trace of it has been found. Within the cairn, two separate chambers were identified roughly 17 metres apart. The north-western chamber, Tomb 1, is a stalled structure divided into three compartments, a type in which upright slabs create internal divisions along the length of the space. Around 40 per cent of its structural stones carry carved designs, including an elaborate pattern of opposed nested Vs, and a radiocarbon date from a female mandible found on the chamber floor places its use to between 3191 and 3009 cal. BC. Later, probably during the Chalcolithic, the tomb appears to have been deliberately closed. A partial female skull, placed upside down at the entry to one of the stalls with adult and juvenile finger bones inside it, dates to between 2492 and 2280 cal. BC, suggesting the roofing was removed and human remains were carefully arranged before the structure was sealed. The south-western chamber, Tomb 2, is broader and more circular, with a possible roofstone bearing cup-marks on its underside and drystone walling that may relate to a later souterrain, an underground passage or storage tunnel of early medieval date, built into the same space centuries afterwards.

A decorated stone bearing passage tomb art, which came to light in 2014 and is thought to derive from this tomb, had already hinted that something significant lay beneath the Hall before the excavations confirmed it. The servants' tunnels discovered below the Georgian house, and the thick-walled semi-circular bowed projection to the south with its internal lime floor and mural passage, added further layers of complexity to a site that had been quietly accumulating history, in very different registers, for over five thousand years.