A souterrain is a dry-stone underground structure, typically consisting of narrow chambers and connecting passages, built during the early medieval period and used variously for storage, refuge, or both. That a rotary quernstone, the kind used for grinding grain by hand, should end up holding up a roof speaks to the casual pragmatism of whoever built or modified this structure in the centuries after its original construction.

The souterrain came to light not through planned archaeological investigation but as a result of road-widening works, excavated under Licence no. C010. What was found beneath the surface were two rectangular chambers linked by a creepway just 2.2 metres long, the kind of connecting passage low enough to require a person to crouch or crawl through. The first chamber was the larger of the two, measuring 12 metres by 1.7 metres and built from substantial blocks and boulders. The second chamber, measuring 2.2 metres by 1.7 metres, extended beyond the limit of excavation, meaning part of it remains unexcavated and its full extent is still unknown. Pottery recovered from the backfill dates to the 13th century, suggesting the structure was in use, modified, or at least still open to deposited material during the medieval period. The site was recorded by O'Connell in 2009.

The souterrain sits outside the boundary of the known ecclesiastical enclosure at Lusk, a detail worth pausing on. Early Irish monastic sites were often surrounded by a roughly circular enclosure, and underground structures associated with them could serve the wider settlement community as much as the religious community itself. There is nothing to see at ground level today, and the site is not publicly presented or marked in any conventional way. Those with a serious interest in the archaeology of north Dublin might approach it through the broader context of the Lusk ecclesiastical complex, which includes a medieval round tower incorporated into a later church still visible in the village.