RTÃ‰ reports that divers from the naval base at Haulbowline will help in assessing the vehicle and establishing if anyone was in it.
The search started after the coast guard received a call from police shortly before 06:00 local time.
The incident happened near the Lee Rowing club.
Valentia Coast Guard, police, a rescue helicopter from Waterford, the fire service and naval divers attended the scene.
A car has been found by divers during a search of the River Lee in County Cork in the Republic of Ireland.