RMS Leinster (2,640 gross tons) was completed in 1897 at Laird Brothers, Birkenhead for the City of Dublin Steam Packet Company. She was one of four named after the Provinces of Ireland – Leinster, Ulster, Munster and Connaught. At the time she was one of the fastest ships at sea with a speed of 24 knots. She operated between Holyhead and Kingstown (now Dun Laoghaire).

At about 9.00 a.m. on 10 October 1918 the RMS Leinster left Carlisle Pier, Kingstown for Holyhead. She was commanded by Captain William Birch (61), a Dubliner who had re-located to Holyhead with his family. The vessel had a crew of 76, drawn from the ports of Kingstown and Holyhead.  Added to these were 22 Dublin Post Office staff working in the ship’s mail sorting room. Of the 680 passengers on board that day, 187 were civilians – men, women and children, mostly from Ireland and Britain.  However, by far the greater proportion of passengers were military personnel, either returning to their units or going on leave. They came from Ireland, Britain, Canada, the United States, New Zealand and Australia.

Shortly before 10.00 a.m. and less than 16 miles from Kingstown, the German submarine UB-123 spotted the Leinster and fired a torpedo that crossed her bows. The submarine’s second torpedo struck the port side, where the postal sorting room was situated. The subsequent explosion blew holes through both her port and starboard sides. Altering course, in an attempted to return to port, the ship, with her speed drastically reduced, began to slowly sink. As lifeboats were being launched another torpedo struck the ship on the starboard side, causing catastrophic damage. The Leinster sank completely soon afterwards, bow first.

Many of those on board were killed in the initial attack. Whilst a number were fortunate enough to clamber into lifeboats, others less fortunate, resorted to clinging onto any pieces of wreckage or flotsam at hand. Sadly many perished whilst awaiting rescue. Eventually naval vessels and other ships arrived and commenced the search for survivors. Those that survived were landed at Victoria Wharf, Kingstown, to receive medical care and comfort.

In the days that followed many more bodies were recovered from the sea. Funerals took place in a number of Irish towns and villages. One hundred and forty-four military casualties were interred at Grangegorman Military Cemetery in Dublin. The recovered bodies of crew members from Holyhead were brought back to the town for burial. Officially, 501 people died in the sinking, making it the greatest ever loss of life in the Irish Sea. However, further research to date has revealed the names of up to 567 casualties.

On the way back to her base UB-123 struck a mine at the North Sea Mine Barrage on 19 October 1918, with the loss of all 36 crew members.