The British Admiralty had several factors to consider when they thought about escorting the mail and rail steamers. For example, the steamers were capable of 21 knots, which was fast, and they could maintain a reasonable speed even in bad weather. In addition, the mail steamers had to meet Post Office crossing times unless the rules were changed. If the mail and rail steamers were to be escorted through the Irish Sea then they would need a destroyer escort capable of 21 knots in bad weather. This would probably mean a modern ship.
Modern US destroyers were used in convoys in December 1917 and January 1918. On two occasions, a single US destroyer escorted a convoy consisting of one City of Dublin Steam Packet Company (CoDSPCo) mail ship and one London and North Western Railway Company (LNWR) express ship. The mail ship carried 1,000 troops and the rail ship was likely to have a similar number. This approach was not pursued, probably because US destroyers began escorting US troop convoys into Bay of Biscay ports from January 1918.
It appears that the British Admiralty remained in command of the escort forces for the incoming and outgoing food convoys passing Queenstown (now Cobh), while the US Navy maintained enough destroyers at Queenstown to provide these escorts. Eventually, the US Navy decided that all their destroyers involved in escorting US troop convoys into the Bay of Biscay ports would be directly controlled by them, not by the British Admiralty.
This decision meant that if the Admiralty wanted to escort the mail and rail steamers then they would have to use British resources. However, available British destroyers were small, old and unlikely to keep up with the larger, more seaworthy CoDSPCo and LNWR ships in bad weather.
Ultimately, the Admiralty decided not to escort the mail and rail boats, but to establish a hunting group of older destroyers based in Holyhead from May 1918. In addition, airships undertook reconnaissance over the route. The effectiveness of hunting patrols in coastal waters has been disputed, and, on the day the RMS Leinster was sunk, bad weather kept the airships on the ground and the old destroyers looking for shelter.
With hindsight, it is difficult to defend the Admiralty’s decision not to escort the fast mail and rail steamers and the slower ships crossing the Irish Sea. The summer months in the final year of the war were the busiest for the CoDSPCo ships. In August 1918 they carried 69,000 passengers in just 31 days and 124 voyages. With a nominal capacity of 650 passengers per voyage, maximum capacity would have been 81,000 passengers per month. It is possible that the LNWR ships may have carried similar numbers. It appears in deciding against a convoy system across the Irish Sea in the final year of the war, the Admiralty had not carried out today’s equivalent of a risk assessment which would have taken the numbers carried and the number of near misses into account. It is even possible that they were unaware of the passenger numbers involved.