When archaeologists moved in ahead of road construction in 2002 and 2003, they came to excavate a rath, the kind of circular earthen enclosure that dots the Irish countryside and typically dates to the early medieval period. What they found instead, pressed into the earth beneath the rath's bank, was something considerably older and stranger.

The excavation, carried out on the northern half of the rath, uncovered a narrow fosse, a ditch, that had been dug long before the rath was ever built. Traced in an arc running west to north-east, the fosse measured between 0.6 and 1 metre wide at the top and no more than 0.8 metres deep, varying in profile from U-shaped to V-shaped. These are modest dimensions, and the archaeologists were clear on one point: whatever this ditch was for, it was not defence. It was too shallow and too slight for that. The structure is thought to be prehistoric in date, most likely associated with a Neolithic habitation site that also underlies the rath. Neolithic occupation in Ireland spans roughly 4000 to 2500 BC, a period when communities were beginning to settle, farm, and mark out the landscape in ways that still puzzle us. The enclosed space, it has been suggested, may have served some ceremonial or ritual function rather than a purely practical one.

What makes the site quietly remarkable is that the story is not finished. The excavation covered only the northern half of the rath, the portion required for the road take. The southern arc of this prehistoric enclosure remains unexcavated, still lying underground beneath the southern side of the rath, waiting. The hillock at Carrowkeel holds at least three distinct moments of human activity stacked within it, and only part of the earliest has ever been seen.