A souterrain, or underground passage, known locally as KE027-001 was discovered sometime in the 1930s or 1940s before being covered over again, leaving behind more questions than answers. While locals know roughly where it lies, running north to south beneath a field boundary between two properties about 300 metres from Brandon Bay, the details of its construction and size remain lost to time.

The area holds particular significance in local folklore, with the small field to the southeast said to be ‘William’s garden’, from which the townland takes its name. Local tradition goes further, claiming this was also the site of William’s castle, though no visible remains survive above ground today. The combination of the hidden souterrain and the legendary castle site suggests this quiet corner of the Magharees Peninsula once held considerable importance, perhaps as a defensive stronghold or a place of refuge during troubled times.

Souterrains are common features across Ireland, typically dating from the early medieval period. These underground structures served various purposes; some were used for storage, others as hiding places during raids, and many likely combined both functions. The fact that this particular example runs beneath a field boundary might indicate it predates the current land divisions, possibly stretching back centuries to when the area’s political and social landscape looked very different. Though covered over and inaccessible today, the souterrain of Garrywilliam remains a tantalising reminder of the hidden history that lies beneath Ireland’s rural landscapes.