A small circular island, barely 30 metres across as measured on the 1840 Ordnance Survey map, it sits 150 metres north-west of the Knockadoon peninsula and carries on its surface the remains of a medieval castle alongside at least three distinct prehistoric settlement sites. That layering of eras, compressed into such a small space, is unusual enough; stranger still is the stone platform at the island's core, a large circular construction around and upon which the castle wall was later built, suggesting that whoever raised the medieval structure was either making deliberate use of something already ancient or simply found good ground and took it.

The site came to formal archaeological attention in 1948 when Professor Frank Mitchell identified it on the rocky foreshore along the island's north-north-east side. His discovery prompted a more systematic excavation in 1956, carried out jointly by Mitchell and Liversage and published in 1958. A narrow trench, six metres long and one metre wide, was cut radially outward from the edge of the stone platform. The finds were modest but telling: heavily patinated flint flakes, a hammer or anvil stone, fire-reddened heat-shattered stone, and animal bone, all recovered from a peaty matrix and sealed beneath a further layer of peaty material. Liversage's interpretation was careful. The shore deposit, he argued, ended at the platform rather than running beneath it, which meant the debris had been washed into place after the platform was already constructed. The island's changing appearance between the 1840 and 1897 Ordnance Survey editions adds another layer of interest: by 1897 the mapped area had grown substantially, to roughly 170 metres north to south and 65 metres east to west, because the lake level had dropped by around half a metre in the intervening decades. Today the lake sits at approximately 75 metres above datum, somewhat higher than in 1897.

Access to Garret Island is made possible by a causeway that runs from the island's southern end south-east to the Knockadoon peninsula. The circular stone platform at the centre of the island remains visible from aerial imagery, recorded as open ground without tree cover in satellite photographs from 2018 and 2020. Visitors approaching from Knockadoon, where extensive Neolithic and Bronze Age remains have been excavated over many years, will find the island a short distance across the water. The castle ruins and the open platform ground are the most immediately legible features; the prehistoric occupation debris, by its nature, lies beneath the surface and within the recorded trenches rather than visible underfoot.