It is a crannog, one of thousands of such structures scattered across Ireland and Scotland, built by driving timber piles and layering stone, brushwood, and peat into shallow water to create a habitable platform. The technology is ancient, with Irish crannogs appearing as early as the Neolithic period and remaining in use well into the seventeenth century. That such a long-lived form of settlement should persist in the west of Mayo is no surprise; the county's countless loughs made the crannog a practical and defensible choice for communities across many centuries.

Béal Deirg Mór itself is a townland in the Erris peninsula, one of the more remote stretches of the western seaboard, where blanket bog and freshwater loughs define the terrain. The area sits within a landscape that has been inhabited since prehistory, and crannogs in this part of Connacht are broadly associated with the early medieval period, roughly the fifth to twelfth centuries, though some were reoccupied or refortified much later. Without specific excavation records or dated finds for this particular site, the precise period of its construction and use remains open. What is clear from its recorded existence is that the lough here was once considered worth building on, and that the effort required to do so was considerable.

The crannog at Béal Deirg Mór is the kind of site that rewards patient looking rather than dramatic discovery. Such islands are often subtle in the water, appearing as a low, slightly too-regular mound, sometimes with a scatter of stones at the waterline or a denser growth of vegetation where organic material has enriched the ground. Visiting the Erris peninsula in summer, when water levels are lower and sight lines across the loughs are clearest, gives the best chance of reading the landscape in a way that makes these features legible.