This ancient souterrain came to light quite by accident when a mechanical digger, working on the park's construction, inadvertently punched through one of its roof slabs. The resulting hole revealed an east-west oriented chamber stretching approximately 3.6 metres in length, with walls built from rough-cut stones using drystone construction techniques. The chamber's design shows considerable skill; the walls rise vertically for about half their height before gradually corbelling inward to support a roof made of five stone lintels.

The souterrain's interior reveals intriguing architectural details that hint at its original purpose. At its widest point, the chamber measures 1.38 metres across and stands 1.53 metres high, providing enough room for a person to move about comfortably. The western end narrows considerably to just 64 centimetres wide, suggesting a possible passageway that extends further, though fallen earth and stones prevented full exploration during the initial discovery. Two recesses carved into the walls, one in the eastern wall and another in the southern wall near where the chamber begins to narrow, may have served as storage alcoves or held lamps to light the dark interior.

Following its documentation by archaeologists, including examination by Judith McCarthy from the Donegal County Museum, the souterrain was carefully covered with soil to preserve it for future generations. Today, no trace of this subterranean structure is visible at ground level in the park, which sits on the north side of St. Conall's Hospital. The chamber remains a hidden piece of Letterkenny's ancient past, its original builders and exact purpose lost to time, though it likely served as a refuge or storage facility during Ireland's early medieval period.