The Naval Service vessel LÉ Aisling is assisting a stricken tanker after it lost power on its main engines west of Loop Head.
The tanker, MV Hellesport Charger, has 22 crew on board.
Weather conditions are described as rough to moderate, with poor visibility.
The tanker is under tow by a tug boat, Thrax, and neither the ship nor the crew are said to be in any immediate danger.
The Naval Service sent the LÉ Aisling to assist the tanker at the request of the Irish Coast Guard, which is co-ordinating the operation.
The stricken vessel is due in the Shannon Estuary between 5pm and 6pm.
The tanker is carrying 15,000 tonnes of caustic soda for the Aughinish Alumina plant there.