Umberto

As an abandoned abbey off the British coast, the fictional Rothermore is analogous to Lindisfarne, although the physical characteristics of Rothermore’s island—stark, steep, and hazardous to landing—are based not on Lindisfarne’s Holy Island but rugged Skellig Michael, located in the Atlantic off the southwestern tip of Ireland. Geographically, there are no islands that correspond to Rothermore’s position off Scottish Aberdeenshire; the closest would be the Orkneys.

As for the abbey itself, Normandy’s Mont Saint-Michel is an obvious source, but it is another monastery named for the same saint that was the primary basis for Rothermore: Sacra di San Michele—inspiration for the abbey in Umberto Eco’s The Name of the Rose—located not on an island but atop a towering pinnacle in the Piedmontese Alps, isolating it as effectively as would the sea (see photo; Elio Pallard, via Wikimedia Commons).