|RMS Leinster Before the War|
The mail ship and ferry RMS Leinster was sunk 10 October 1918 in the Irish Sea with little over a month left in the Great War. The ship was torpedoed and sunk by German submarine UB-123 shortly after leaving Dún Laoghaire (then Kingstown). Current research shows that 564 people were lost in the sinking. It was the greatest loss of life in the Irish Sea and the highest death toll on an Irish-owned ship.
On 10 October 1918, the [ship's] luck ran out when, shortly after leaving Dún Laoghaire, German submarine UB-123 fired three torpedoes at the RMS Leinster. The first missed, but the other two struck and sank the ship. A total of 21 of the 22 postal sorters aboard were lost, as were about half of the ship’s crew. Among the lost were civilians from various parts of Ireland and Britain. That day the majority of passengers—and the majority of casualties—were military personnel going on leave or returning from leave. They included soldiers, sailors, airmen, and military nurses from Ireland, Britain, Australia, New Zealand, Canada, and the United States. Almost 150 of them are buried in Grangegorman Military Cemetery in Dublin.
On 18 October 1918, while returning to Germany, UB-123 struck a mine in the North Sea. Captain Robert Ramm and all of his young crew were lost
The sinking had a major impact on the peace negotiations that were in process at the time. On 14 October U.S. president Woodrow Wilson sent a note to the German government stating, among other things, that there could be no peace as long as Germany attacks passenger ships. A week later, on 21 October 1918: Reinhard Scheer, admiral of the German High Seas Fleet, signaled his submarines "To all U-boats: Commence return from patrol at once. Because of ongoing negotiations any hostile actions against merchant vessels prohibited. Returning U-boats are allowed to attack warships only in daylight. End of message. Admiral."
Sources: The Irish Times, RMS Leinster Website