The hollow runs roughly six metres along its longer east-west axis, with a shorter four-metre arm extending north-south from its south-east end. It is not a dramatic sight by any measure, but a large limestone upright standing about nine metres to the south has long been pointed to by local tradition as the original entrance stone, a piece of oral memory that has outlasted whatever roofing once covered the passage below.

The ringfort in which this souterrain sits is a separate monument in its own right, and souterrains of this kind are commonly found within such enclosures across Ireland, suggesting a close functional relationship between the two. The Kilnalappa example was noted by Neary in 1914, and the two displaced boulders near the western end hint at a structure that has shifted and partially collapsed over the intervening centuries. The L-shaped plan is a recognised souterrain form, the change in direction possibly intended to impede intruders or to create distinct sections for different uses. Without excavation it is difficult to say more about its date or precise character, but the ringfort context points broadly to early medieval occupation of the site.