The River Derreen (Irish: An Daoirín) is a large river in the southeast of Ireland. It rises on central Lugnaquilla Mountain in the western Wicklow Mountains and flows south from Lugnaquilla Mountain and then southwest to join the River Slaney south of Tullow, passing close to Hacketstown and Tullow, Co. 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.