At the mouth of the river then known as the Gaillme, and today called the Corrib, Irish annalists recorded the existence of a dún, a native fortification or stronghold, as early as 1121. The place appears under a cluster of variant names across twelfth and thirteenth-century sources, all translating roughly as the castle or fortification at the mouth of the river Galway. What is unusual about this site is not merely its age, but its near-total elusiveness: no definitive physical evidence for the structure has yet been found.

The annals trace a turbulent career for the dún. It was demolished in 1132, again in 1149, and burned in 1161, yet it kept being rebuilt, drawn back into the fluctuating wars between Connacht and Munster. A curious detail survives from 1178: when the river unexpectedly ran dry that year, the inhabitants of the dún took the opportunity to collect old objects from the exposed riverbed. The fortification disappears from the written record after that until 1230, when Richard de Burgo marched to what the annals call Caisleán Bhun na Gaillimhe and laid siege to it. The Connacht forces, under Odo O'Flaherty and Odo, King of Connaught, held the west bank of the river; de Burgo's men pressed from the east. The seventeenth-century historian Roderic O'Flaherty, drawing on these same annals, recorded that the besiegers eventually withdrew without achieving their aim. It is the only source that explicitly places the dún on the east bank. De Burgo returned in 1232 and this time succeeded, erecting his own castle on the site. Excavations at Quay Lane in 2017 uncovered what appears to be part of that thirteenth-century de Burgo structure, lending some weight to the idea that the Norman foundation was laid directly over the Irish one.