Lough Allen (Irish: Loch Aillionn) is a lake on the River Shannon in northeastern Connacht, Ireland. Most of the lake is in County Leitrim, with a smaller part in County Roscommon. The lake lies to the south of the River Shannon's source, near the Iron Mountains, and is the uppermost of the three main lakes on the river. The other two, Lough Ree and Lough Derg are much further to the south.The lake is shaped like an upturned isosceles triangle.