Garret Island, a small outcrop roughly 150 metres northwest of the Knockadoon peninsula, carries traces of Neolithic and Bronze Age habitation on its surface and, more remarkably, on shallow submerged reefs to its south. Flint artefacts recovered from the lake bed in this area suggest that people once moved and worked across ground that is now beneath the water. The island's size, as recorded on maps, shifts noticeably between the nineteenth-century Ordnance Survey editions: the 1840 six-inch map shows a compact circular island around 30 metres in diameter, while the 1897 twenty-five-inch map depicts a far larger landmass, roughly 170 metres north to south and 65 metres east to west, reflecting a lake level that was about 1.5 metres lower at that point. The lake today sits at approximately 75 metres OD, similar to the 1840 reading, and excavations have suggested the water was even lower in prehistory.

The archaeological story here involves several decades of careful fieldwork. Professor Frank Mitchell identified prehistoric and palaeo-environmental deposits on the island in 1948, and in 1956 he returned with Liversage to excavate them, publishing their findings in 1958. The flint scatter on the island surface and on the lake bottom to its south was largely gathered by Seán P. Ó Ríordáin during his broader campaign of excavation across Knockadoon in the 1950s, where extensive Neolithic and Bronze Age monuments have since been documented in detail, most recently by Cleary in 2018. Garret Island actually holds three distinct prehistoric settlement sites, all considered part of the same broader monument, and these cluster around a medieval castle that also occupies the island. A stone causeway runs from the island's southern end southeast toward the Knockadoon shore, suggesting the island was routinely connected to the peninsula rather than treated as a remote or isolated spot.

The submerged reefs to the south of the island are not always visible; their appearance depends on both the season and the quality of light. A Google Earth image taken in late June 2018 captures them clearly, while one taken in September 2020 shows nothing. Summer visits, when lake levels tend to drop slightly and sunlight strikes the water at a useful angle, offer the best chance of seeing the shallow reef structure from the shore or from the causeway approach. The causeway itself remains a useful way to read the relationship between island and peninsula, and the medieval castle ruin provides an obvious focal point once on the island. What repays closer attention, though, is the ground underfoot and the water immediately to the south, where the lakebed holds scattered flint that once belonged to a landscape considerably drier than the one visible today.