The Barrow is a river in Ireland. It is one of The Three Sisters; the other two being the River Suir and the River Nore. The Barrow is the longest of the three rivers and, at 192 km (120 mi), the second-longest river in Ireland, behind the River Shannon. The catchment area of the River Barrow is 3,067 km2 before the River Nore joins it a little over 20 km before its mouth. The river's long term average flow rate, again before it is joined by River Nore, is 37.4 cubic metres per second. At the merger with the River Suir, its catchment area is ca. 5,500 km2 and its discharge over 80 m3/s.

The River Suir is a river in Ireland that flows into the Atlantic Ocean through Waterford after a distance of 185 kilometres (115 mi). The catchment area of the Suir is 3,610 km2. Its long term average flow rate is 76.9 cubic metres per second (m3/s), about twice the flow of either the River Barrow or the River Nore before these join, but a little less than the Barrow's flow when it meets the Suir 20 km downstream.