In the sorting room below decks, the postal clerks of the Dublin and Holyhead mail service were already at work on the mails when the Leinster left Kingstown that morning, bound for Holyhead as she was most days. UB-123 torpedoed her about four nautical miles east of the Kish light, and roughly 564 people died — passengers, crew, soldiers going back, and nearly all of the sorters. The Armistice was a month off. She was a scheduled sailing, and she is still the greatest loss of life in the Irish Sea.
UB-123 struck a mine in the Northern Barrage on her way home that same month and went down with everyone aboard, so there's no log, no patrol report, nothing from her side. What we know of those few minutes comes almost entirely from the people who survived the Leinster.
The sorting room held 22 men that morning and one came out of it alive — John Higgins, I believe. The mails were handled by a small permanent staff who did the Kingstown to Holyhead run day after day; that was 1918, and most of them were known to each other for years.