Coarse Island, measuring roughly 65 metres north to south and 38 metres east to west, sits about 230 metres from the eastern shoreline, or at least it did when the Ordnance Survey mapped it in 1838 and again in 1930. Since then, a drop in the lake's water levels has exposed enough surrounding land to absorb it into the shore, and the scrub that has colonised the area now makes it impossible to say with any certainty where the island once ended and the lake bed began.

The site appears in heritage records as a possible crannog, which is an artificial or artificially modified island, typically constructed during the early medieval period from layers of timber, stone, peat, and brushwood, and used as a defended lake dwelling. The designation, however, remains tentative. No timber remains have been found, and there are no visible accumulations of stone that would confirm deliberate construction. What adds a further layer of interest is that the 1838 Ordnance Survey map shows a causeway-like feature connecting the island to the mainland to the northwest. Such a causeway would be consistent with crannog use, as these sites were often linked to the shore by narrow, sometimes deliberately submerged, approaches designed to control access. That feature too has since been absorbed into the surrounding dry land and is now overlain by a modern road or causeway, meaning the historical trace survives only in the cartographic record.