Lough Beagh is about northwest of Letterkenny. It measures about long and wide and lies in the narrow Glenveagh valley surrounded by the Derryveagh and Glendowan Mountains. Steep granite cliffs rise on both sides of the lake to heights of about. The lake has numerous small islands at its northern end.
Hydrology
Lough Beagh is fed mainly by the Owenbeagh Riverentering at its southern end. The lake drains northwards into the Owencarrow River. The Owencarrow connects the lake with its similar northern neighbour, Glen Lough.