Look more carefully, though, and the island conceals something far older beneath its waterline. A large circular stone platform, around which a medieval castle wall was later built, turns out to be the physical trace of prehistoric settlement activity stretching back to the Neolithic period, layered under centuries of rising lake water and medieval construction. The island sits just 150 metres north-west of the Knockadoon peninsula, itself one of the most archaeologically dense areas in Ireland, with Neolithic and Bronze Age monuments recorded across its length.

The site came to scholarly attention in 1948, when Professor Frank Mitchell spotted its potential on the rocky foreshore of the island's south-south-east side. His discovery prompted a full archaeological and palaeo-environmental excavation in 1956, carried out by Mitchell and Liversage, whose findings were published in 1958. The excavations opened trenches on two sides of the island. At Site B, a trench extending outward from the edge of the circular stone platform produced flint artefacts and flakes, worked and butchered animal bone, prehistoric pottery from both the Neolithic and Bronze Age periods, and fire-reddened heat-shattered stone, the kind of debris associated with cooking or craft activity. A layer of tan clay beneath a cobbled surface survived intact and was interpreted as an in-situ Neolithic deposit. Fossilised tree stumps recorded at the buried lake margin added a further dimension to the story: a radiocarbon sample from one stump yielded a calibrated date of around 1722 BC, suggesting the old ground surface was gradually swallowed by rising lake water during the Bronze Age. The island's changing size across the Ordnance Survey maps of 1840 and 1897 reflects exactly this fluctuation; the lake level in 1840 was recorded at 76.8 metres OD, while by 1897 it had dropped by roughly 1.5 metres, exposing a much larger landmass.

Garret Island is accessible via a causeway running from the island's southern end south-east to the Knockadoon Peninsula shore, making it reachable on foot when conditions allow. The circular platform that anchored the prehistoric settlement, and later the castle, is visible as an area of open ground without tree cover and can be made out on aerial imagery. Three separate prehistoric settlement sites are recorded on the island in total, and the remains of the medieval castle sit directly on top of the same ancient platform, a quiet collision of eras that repays a slow walk around the foreshore rather than a quick glance from the causeway.