Rat Island sits in waters that are unusually clear even by Irish standards, a consequence of the marl-rich limestone geology that gives Lough Carra its distinctive milky-turquoise colouring. The lake is one of a chain of connected loughs in the area, and its islands have attracted human attention for thousands of years, with evidence of early settlement scattered across the surrounding landscape.

Lough Carra itself has long associations with Irish cultural and literary life. The writer George Moore, who grew up at Moore Hall on the lake's eastern shore, wrote about these waters with evident affection, and his ashes were interred on Castle Island nearby. The lake is also known to archaeologists for the remains of crannogs, which are artificial or modified islands constructed from timber, stone, and brushwood, typically used as defended dwelling places from the Bronze Age through to the early medieval period. Whether Rat Island is natural, modified, or has any recorded archaeological feature associated with it remains, for now, a matter that the formal record has yet to fully illuminate. The name itself is likely a plain descriptive one, of the kind that proliferates across Irish lakelands, where islands were named for the wildlife found on them rather than for any grander associations.

The lake is accessible from several points around its shoreline, and the surrounding area rewards patient exploration. The water's exceptional clarity makes it possible, on calm days, to see the lake bed at considerable depth from a boat, the pale marl giving the surface an almost Mediterranean quality that catches visitors off guard in a Connacht autumn.