Lough Neagh is a freshwater lake in Northern Ireland and is the largest lake on the island of Ireland and in the United Kingdom. It has a surface area of 151 square miles and is about 19 miles (31 km) long and 9 miles (14 km) wide. According to Northern Ireland Water, it supplies 40.7% of Northern Ireland's drinking water. Its main inflows are the Upper River Bann and Blackwater, and its main outflow is the Lower Bann. There are several small islands, including Ram's Island, Coney Island and Derrywarragh Island. The lake bed is owned by the 12th Earl of Shaftesbury and the lake is managed by Lough Neagh Partnership. Its name comes from Irish Loch nEachach, meaning "Eachaidh's lake".

The River Bann is the longest river in Northern Ireland, its length, Upper and Lower Bann combined, being 129 km (80 mi). However, the total length of the River Bann, including its path through the 30 km (19 mi) long Lough Neagh is 159 km (99 mi). Another length of the River Bann given is 90 mi. The river winds its way from the southeast corner of Ulster to the northwest coast, pausing in the middle to widen into Lough Neagh. The River Bann catchment has an area of 5,775 km2. The River Bann has a mean discharge rate of 92 m3/s. According to C. Michael Hogan, the Bann River Valley is a settlement area for some of the first human arrivals in Ireland after the most recent glacial retreat.