RMS Leinster, a ship operated by the City of Dublin Steam Packet Company and serving as the Kingstown–Holyhead mailboat, is torpedoed and sunk by the Imperial German Navy submarine SM UB-123, which is under the command of Oberleutnant zur See Robert Ramm, just outside Dublin Bay on October 10, 1918, while bound for Holyhead. The exact number of dead is unknown but researchers from the National Maritime Museum of Ireland believe it to be at least 564, making it the largest single loss of life in the Irish Sea.

In 1895, the City of Dublin Steam Packet Company orders four steamers for Royal Mail service, named for four provinces of Ireland: RMS Leinster, RMS Connaught, RMS Munster, and RMS Ulster. The RMS Leinster is a 3,069-ton packet steamship with a service speed of 23 knots. The vessel, which is built at Cammell Laird‘s in Birkenhead, England, is driven by two independent four-cylinder triple-expansion steam engines. During World War I, the twin-propellered ship is armed with one 12-pounder and two signal guns.

The ship’s log states that she carries 77 crew and 694 passengers on her final voyage under the command of Captain William Birch. The ship had previously been attacked in the Irish Sea but the torpedoes missed their target. Those on board include more than 100 British civilians, 22 postal sorters and almost 500 military personnel from the Royal Navy, British Army and Royal Air Force. Also aboard are nurses from the UK, Ireland, Australia, New Zealand, Canada and the United States.

Just before 10:00 AM as it is sailing east of the Kish Bank in a heavy swell, passengers see a torpedo approach from the port side and pass in front of the bow. A second torpedo follows shortly afterwards, and strikes the ship forward on the port side in the vicinity of the mail room. Captain Birch orders the ship to make a U-turn in an attempt to return to Kingstown as it begins to settle slowly by the bow. It sinks rapidly, however, after a third torpedo strikes her, causing a huge explosion.

Despite the heavy seas, the crew manages to launch several lifeboats and some passengers cling to life-rafts. The survivors are rescued by HMS Lively, HMS Mallard and HMS Seal. Among the civilian passengers lost in the sinking are socially prominent people such as Lady Alexandra Phyllis Hamilton, daughter of James Hamilton, 2nd Duke of Abercorn, Robert Jocelyn Alexander, son of Irish composer Cecil Frances Alexander, Thomas Foley and his wife Charlotte Foley (née Barrett) who was the brother-in-law of the world-famous Irish tenor John McCormack. The first member of the Women’s Royal Naval Service to die on active duty, Josephine Carr, is among the dead, as are two prominent officials of the Irish Transport and General Workers’ Union, James McCarron and Patrick Lynch.

Captain Birch who is wounded in the initial attack, drowns when his lifeboat is swamped in heavy seas and capsizes while attempting to transfer survivors to HMS Lively. Several of the military personnel who die are buried in Grangegorman Military Cemetery.

Survivors are brought to Kingstown harbour. Among them are Michael Joyce, an Irish Parliamentary Party MP for Limerick City, and Captain Hutchinson Ingham Cone of the United States Navy, the former commander of the USS Dale (DD-4).

One of the rescue ships is the armed yacht and former fishery protection vessel HMY Helga. Stationed in Kingstown harbour at the time of the sinking, she had shelled Dublin during the 1916 Easter Rising in Dublin two years earlier. She was later bought and renamed the Muirchú by the Irish Free State government as one of its first fishery protection vessels.

At October 18, 1918 at 9:10 AM SM UB-125, outbound from Germany, picks up a radio message requesting advice on the best way to get through the North Sea minefield. The sender is Oberleutnant zur See Robert Ramm aboard SM UB-123. Extra mines have been added to the minefield since SM UB-123 had made her outward voyage from Germany. As SM UB-125 had just come through the minefield, Vater radios back with a suggested route. SM UB-123 acknowledges the message and is never heard from again.

The following day, ten days after the sinking of the RMS Leinster, SM UB-123 accidentally detonates a mine while trying to cross the North Sea and return to base in Imperial Germany. It is October 19, 1918. Oberleutnant zur See Robert Ramm, who has a wife and children, never returns to them. Thirty-five other German families are similarly bereaved. No bodies are ever found.

In 1991, the anchor of the RMS Leinster is raised by local divers. It is placed near Carlisle Pier and officially dedicated on January, 28, 1996.