The reservoir was created in the 1940s when the River Liffey was dammed to supply Dublin with water, submerging the village of Old Booleigh and much of the surrounding farmland. The graveyard, enclosed by a modern wall and measuring no more than 35 metres at its widest, predates that flooding by at least two centuries; the earliest legible grave marker here dates to 1747.

What makes the site genuinely curious is the possibility that it is far older than those eighteenth-century headstones suggest. A curving dip in the ground along the north-east side of the enclosure may trace the edge of an earlier circular enclosure, roughly 65 metres in diameter. Circular ecclesiastical enclosures of this kind are a well-recognised feature of early medieval Irish Christianity, typically marking the outer boundary of a monastic or church site founded somewhere between the sixth and twelfth centuries. The shape is often the only surface evidence that survives. Here, that faint arc in the ground hints at a sacred geography that the later rectangular graveyard quietly absorbed. A holy well lies to the east of the site, and holy wells in Ireland frequently signal long continuity of religious practice at a location, often predating formal church organisation altogether.