This detail, small and easy to overlook in an excavation report, is what makes the building they belonged to genuinely puzzling. The shells were not food waste casually discarded; their positioning suggests they were put there on purpose, either as a dedicatory deposit of some kind or as part of a domestic ritual whose meaning has not survived.

The building, catalogued as Structure 9.1, came to light during excavations focused on the middle enclosure of Dún Aonghasa, the great prehistoric cliff-fort that dominates the southern edge of Inis Mór. It sat immediately to the south-west of the entrance passageway, and what remained of it was an irregular post-hole trench, eight metres long and curving from south-west through north to south-east, enclosing an area roughly 4.2 metres by 3.6 metres. The curve of the trench points strongly to an oval or circular structure, built from upright posts set into the ground, though no trace of a doorway survived. A paved pathway extending from the north-west of the trench offers the most plausible candidate for where an entrance once stood. The interior sloped noticeably from west to east, and beyond two shallow gullies, nothing of the floor archaeology remained. Charcoal recovered from one of the post-holes returned a radiocarbon date of between 400 and 200 cal. BC, placing the structure firmly in the Iron Age. Whether it was a dwelling is genuinely uncertain. Its position directly inside the entrance to the fort has led researchers to consider that it may instead have served some ceremonial function, connected to the rituals of passing in and out of an enclosed, significant space. A building at a threshold, in other words, whose purpose was less about shelter than about marking passage.