It rises on the southern slopes of Lybagh and Slievemaan and flows southwest until it joins the River Slaney in County Carlow,[2] 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.

The River Derreen holds occasional salmon in the spring but the upper reaches are better known for trout, averaging 0.5lbs.

There are two angling clubs with fishing rights on the Derreen River and the remainder is in private hands.