The depression, roughly seven metres north to south and nearly nine metres east to west, sinking about a metre into the earth, sits just inside the inner bank at the south-east. It is the collapsed surface trace of a souterrain, an underground stone-lined passage or chamber of the kind that early medieval Irish communities built beneath and beside their enclosed settlements, most likely for storage, refuge, or both. The structure itself is gone from view, folded into the soil, leaving only this sunken outline as evidence that something was once carefully constructed beneath the surface.

The hollow was first recorded by a researcher named Bowman in 1934, who noted a depression on the south part of the site and interpreted it as marking the position of a souterrain. The ringfort it belongs to is a separate, catalogued monument, and the souterrain would have been an integral feature of that enclosure during its period of use. Ringforts, the circular earthen or stone enclosures that dot the Irish countryside in their thousands, were the dominant settlement form of early medieval Ireland, in use roughly between the fifth and twelfth centuries. Souterrains were a common addition, sometimes elaborate in their construction, with corbelled roofs and multiple chambers, though at Island what remains is too disturbed to say much about the original form.