Beneath the foundations of what would become one of Ireland's most visited prehistoric forts, excavators found a narrow hollow cut into the bedrock, roughly seven metres long and less than a metre wide, quietly packed with the residue of daily life from the early Iron Age or late Bronze Age. It is the kind of discovery that reframes a well-known monument entirely.

The hollow is the southernmost of three such features uncovered along the western side of Dún Aonghasa's inner enclosure. It tapers off towards the cliff face to the south, while its northern end angles westward and disappears beneath the phase one enclosure wall, lying 1.3 metres below the wall terrace foundations. That depth tells its own story: whoever occupied this spot predated the fort's construction by a significant margin. A radiocarbon date taken from animal bone within the hollow's primary deposit placed the occupation between 910 and 800 cal. BC. The deposit itself was a thin, compact layer of dark brown clay flecked with charcoal, containing scattered animal bone alongside limpet and periwinkle shells, the kind of shellfish remains that suggest casual, everyday eating rather than any ceremonial purpose. Among the finds were pottery sherds, fragments of bone pins, a piece of pumice, and a hammerstone. No walls or postholes were identified, but two small spreads of paving in the layers above the hollow hint that a structure may once have stood here. Most striking was a hoard of four hollow bronze rings with lateral buffers, found clustered in the south-western area of the hollow. Objects of this kind, deliberately deposited together, are generally interpreted as hoards in the archaeological sense, whether set aside for safekeeping or placed with some more deliberate intention that is now impossible to recover. The combination of domestic debris and a small bronze hoard within the same confined space, sealed beneath the walls of a fort built centuries later, gives the site an unusual density of meaning for something so physically slight.