A souterrain, which is a stone-lined subterranean tunnel or chamber built typically in early medieval Ireland for storage or refuge, was discovered here at Garrywilliam sometime in the 1930s or 1940s, and was subsequently covered over again. No record of its dimensions or construction survives. What is known is that it ran beneath a north-to-south field boundary separating two properties, on level ground roughly 300 metres from the shore of Brandon Bay.

The townland name itself carries a quiet historical riddle. The small field to the south-east is locally said to be the garden of a figure known as William, and it is from this garden that the placename Garrywilliam is thought to derive, "garry" being an anglicisation of the Irish "garrái", meaning garden or enclosed plot. Local tradition goes further, placing William's castle somewhere in the same vicinity, though no physical trace of it has been identified. The combination of a named individual, a garden, a castle, and a subterranean passage in such close proximity suggests a layered occupation of this strip of land, though the details remain stubbornly vague. The tradition was recorded as local knowledge as early as 1951.