Its western entrance is barely passable, measuring just sixty centimetres long and twenty centimetres high, and lies nearly a metre below ground level. A second entrance, located nine metres to the north-west beside a tree, is now blocked entirely with debris. What makes this particular structure unusual is not its age but its afterlife: local tradition holds that the passage was used by smugglers, suggesting it served a very different purpose in more recent centuries than the one for which souterrains were originally built.

Souterrains are earth-cut or stone-lined underground chambers and passages, typically associated with early medieval Irish settlement sites, where they were used for storage and possibly as places of refuge. The example at Derreen follows the basic form, with an earth-cut tunnel and a curving roof, but its smuggling association places it within a much later chapter of Kerry's history. The harbour at Kilmakilloge, tucked into the Beara Peninsula, would have made an ideal landing point for illicit goods in the eighteenth and nineteenth centuries, when the Kerry coastline was well known for such activity. The most specific account of the interior comes from the Marquis of Lansdowne, who noted in 1937 that it was possible to 'sit up' inside the passage, suggesting a space of modest but usable dimensions rather than a simple crawlway.

The entrance on the western side of the path remains partly accessible, though its small dimensions make any investigation difficult. The location on Grinneen Island, reached through a wooded area, means the site is easy to overlook entirely without local knowledge; indeed, the second entrance was only identified after someone familiar with the area pointed it out.