RMS Leinster was an Irish ship operated by the City of Dublin Steam Packet Company. She served as the Kingstown-Holyhead mailboat until she was torpedoed and sunk by the German submarine UB-123, which was under the command of Oberleutnant zur See Robert Ramm, on 10 October 1918, while bound for Holyhead. She sank just outside Dublin Bay at a point 4 nautical miles (7.4 km) east of the Kish light. The exact number of dead is unknown but researchers from the National Maritime Museum believe it was at least 564; this would make it the largest single loss of life in the Irish Sea.