The river begins at Lough Fergus in north Clare and flows into the Shannon Estuary.

There is also a small branch which splits off just north of Ennis and rejoins the main flow to the east of the town.

[9] The River Fergus is noted for its trout and salmon fishing.

[2] A water-powered flour mill was located at Clifden, Corofin, just after the river exits Lough Inchiquin.

[13] The river's name is recorded in old Irish manuscripts as Forgas and Forgus.