A souterrain, to use the Irish archaeological term, is an artificial underground passage or chamber, typically associated with early medieval settlement and thought to have served as storage space, a place of refuge, or both. What makes the Dunsink example quietly interesting is the qualifier attached to it in the records: it is described as a possible rock-cut souterrain, meaning the evidence, though suggestive, left room for uncertainty even after investigation.

The two exposed passages together stretched to around twelve metres in length, with a height ranging from 1.2 to 1.67 metres and a width of between 0.7 and 0.8 metres, dimensions narrow enough to require a person to move through carefully. Excavations followed in 1981, led by a team whose findings were subsequently written up by ÓhEailidhe in 1982. The dig produced two hammerstones, small hand-held stones used for striking or shaping other materials, a modest find that nevertheless confirms human activity at the site. The results were later referenced by Clinton. The site was compiled into the archaeological record by Geraldine Stout, whose work has been central to documenting prehistoric and early medieval monuments across the Dublin and Meath region.

Dunsink is perhaps best known to most people as the location of the old observatory established in the eighteenth century, which gives the area a certain institutional familiarity. The souterrain, by contrast, is an altogether quieter presence. Access to the precise location is not straightforward for casual visitors, as the feature was exposed during agricultural drainage works rather than as part of a publicly accessible excavation. Those with a particular interest in early medieval underground structures would do better to consult the published excavation report or the Sites and Monuments Record for County Dublin before making any attempt to locate it on the ground.