The feature sits in the north-north-west sector of a ringfort, the circular earthen enclosure it once served, and what survives today is not a dark tunnel to crawl through but a shallow depression in the grass, tracing the outline of what once lay beneath.

The hollow follows two axes: a longer run of 8.5 metres on a west-north-west to east-south-east alignment, which then turns sharply to the north-north-east for a further 4.7 metres, giving the whole feature its distinctive L-shape. It averages about 3.5 metres in width and just 0.65 metres in depth, the roof having long since collapsed inward and the structure gradually filling with soil and vegetation. Souterrains of this kind are found across Ireland in association with ringforts, the latter being the enclosed farmsteads of early Christian Ireland, occupied roughly between the fifth and twelfth centuries. The underground passages connected to them were likely used to store perishable goods in cool, stable conditions, and in some cases to provide a place of concealment during raids. The Chelsea example, tucked into the interior of its parent enclosure, fits that pattern well enough, even if the details of its construction and use remain unrecorded.