In the fields around Hallahoise in County Kildare, there is an ancient enclosure that no one walking the land would easily notice. It has no visible walls, no earthen banks, no obvious outline. What gives it away is the grass itself, or more precisely, the way crops grow differently over buried features, producing what archaeologists call cropmarks, subtle variations in colour and height that become legible only from the air.

On 30 May 1990, Dr. Gillian Barrett was conducting an aerial photographic survey when she captured the telltale pattern in a photograph catalogued as GB90.AV.01. The image revealed an irregular, triangular enclosure defined by a fosse, a ditch dug to mark or defend a boundary, its shape pressed into the soil so long ago that the ground has long since levelled over it. What makes the site particularly curious is that it does not stand alone. Roughly 400 metres to the south-west, another triangular enclosure of apparently similar character has also been identified. Triangular enclosures are unusual in the Irish archaeological record, where circular forms are far more common, and having two such enclosures in close proximity raises questions that the aerial photographs alone cannot answer. Whether they were contemporary, connected, or simply coincidental neighbours is unknown.