The Glencullen River, often called the Cookstown River below the village of Enniskerry, is an approximately 11-kilometer watercourse flowing through the scenic landscapes of southern County Dublin and northern County Wicklow. It originates in the Glencullen/Glendoo valley at an elevation of 442 meters and winds its way through the Knocksink Wood nature reserve before joining the River Dargle near Bray. The river's geology is characterized by a fault line, with its waters flowing primarily over Ordovician schist bedrock and glacial deposits, occasionally cutting down to the bedrock itself. This creates a varied and interesting environment for anglers.