A collapsed entrance, sitting at the base of a shallow oval depression roughly two and a half metres long and just under a metre deep, opens into an underground passage running northward beneath the field. The roof lintels are still in place, resting on stone side-walls, which means that whatever lies beyond the collapsed mouth has retained something of its original form, sealed away beneath the soil.

The structure is a souterrain, a type of man-made underground passage or chamber associated in Ireland primarily with the early medieval period, roughly the sixth to the twelfth centuries. They are commonly found within raths, which are the circular or oval earthwork enclosures that served as farmsteads during that era, enclosed by banks and ditches for both status and security. Souterrains are generally understood to have served as cool storage spaces, places of refuge, or both. The one at Deelis sits in the southern quadrant of its rath, a detail that may reflect deliberate planning, since souterrain entrances were often positioned to make forced entry awkward. The passage runs north from the collapsed opening, suggesting a layout that would have extended well into the interior of the enclosure.