The River Derreen is a large river in the southeast of Ireland. It rises on the southern slopes of Lybagh and Slievemaan and flows southwest until it joins the River Slaney in County Carlow, south of Tullow, passing close to Hacketstown and Tullow, County Carlow before it joins the Slaney upstream of Aghade bridge. Running through tillage and pastureland in its lower reaches, the Derreen with its sandy, gravelly bottom is a prime salmon spawning tributary of the Slaney. Passing through counties Wicklow and Carlow, it is the first large, major tributary of the River Slaney.