Bush Island is a crannog, an artificial island constructed by piling stones onto a lakebed, a technique used across Ireland and Scotland from the Bronze Age well into the medieval period, typically to create defensible or otherwise advantageous dwelling places in the middle of water. The platform measures roughly fifteen metres north to south and only about two metres east to west, a narrow, elongated shape that speaks to deliberate, effortful construction rather than anything left by geology.

Lough Rea holds an unusual concentration of these structures. Bush Island is one of six crannogs identified in this part of the lake alone, clustered together in a way that suggests the lough was once a place of some social or strategic significance, with communities choosing, over generations or centuries, to build out into the water rather than settle the surrounding land. The island today is marked by a few trees growing from the stone pile, an unremarkable silhouette from the shore that gives almost nothing away about what lies beneath.