At approximately 2.5 miles long and 0.5 miles wide, Loch Na Fooey is set in a steep-sided valley, surrounded by the mountains of Galway to the south and Mayo's Partry Mountains to the north. A number of rivers feed the lake, including the River Fooey (), which rises from the Devil's Mother mountain and enters the lake at the western shore. The Finny River, rising from the south-east of the lake drains into the southwest part of Lough Mask. Also many mountain streams flow into the lake. The lake has a small sandy shore on its western end.