The Black River at Shrule is a limestone-fed river that flows along the Galway-Mayo border, passing through the village of Shrule before emptying into Lough Corrib near Greenfields, north of Headford. Its limestone geology makes it a highly productive waterway, generating excellent insect hatches that sustain strong populations of brown trout. The river's character is shaped significantly by its fertile, nutrient-rich waters, which promote lush aquatic weed growth as the season progresses — a defining feature that heavily influences when and how anglers approach it. Spring is widely regarded as the prime time to fish the Black, as the river fishes most effectively before weed growth becomes dense and low summer water levels make angling difficult.