Lough Allen is in County Leitrim, with a small part in County Roscommon. The lake lies to the south of the River Shannon’s source, The Shannon Pot. It is unnoticed by Sliabh an Iarainn and is the largest of the three lakes on the River Shannon.
The Shannon enters the lake from the north and leaves the lake at the southern end. The R280 local road runs along the western side of the lake, while the R207 trails the eastern shore, from Ballinagleragh to Drumshanbo. The R200 road is on the northern side of the lake, travelling east from Drumkeeran to Dowra.
The construction of the Shannon hydroelectric scheme in the early 1900s, the lake became a storing tank for the power station in Limerick nearly 100 miles away, with channels to control the current into the river. This helps to preserve the flow during dry stages and accomplish flooding at other times. The Lough Allen Canal was developed by the ESB, which was hardly used, until its refurbishment in 1996. It is now used as a service area where the trails have been developed for walking and cycling along the Shannon Blueway.