Souterrains, which are man-made tunnels or chambers typically constructed during the early medieval period, are found across Ireland in considerable numbers, yet each one carries its own quiet mystery. Built from dry-stone walling and roofed with large capstones, they were dug beneath or beside settlements, most likely for the storage of food and valuables, though some may also have served as places of refuge during raids. The fact that one exists at Castlegar tells us that this corner of Mayo was once a settled, inhabited place of some consequence, even if the community that built it left no other obvious trace above ground.

Beyond the bare fact of its existence and its location within the townland, the specific history of this particular souterrain, its dimensions, its state of preservation, and any associated finds or structures, remains difficult to establish from what is currently in the public domain. The site is a recorded monument, which means it has been identified and catalogued as part of Ireland's archaeological heritage, but the details that would bring it fully to life, names, dates, excavation history, are not yet widely accessible. What is clear is that souterrains of this type generally date to somewhere between the sixth and twelfth centuries, a period when the Irish countryside was organised around ringforts and small farming settlements, and when the need to protect stored goods was a practical and pressing concern.