Derry Island is one of these, its name derived from the Irish word for oak wood, doire, which suggests that at some point the island carried a canopy of trees, though what remains today tells a more fragmented story.

Lough Mask and the islands scattered across it have long histories of ecclesiastical and secular occupation. The lake sits within a limestone karst landscape, where water drains unpredictably through underground channels rather than surface rivers, and the islands offered both natural protection and a degree of isolation that appealed to early monastic communities and later to those seeking defensible ground during periods of conflict. Many of the islands in this part of Connacht retain earthworks, enclosures, or the foundations of early Christian structures, and the name Derry, with its oak-wood associations, places the island within a wider tradition of sacred or significant woodland sites that recur throughout early Irish topography.