Gents:

The 2 boats that were ordered by Eire in May 1939 were Thornycroft yard numbers 2463 and 2464.
Estonia ordered 1 boat from Thornycroft (which possibly was already started as a private venture in 1938).
So far, I have not seen any evidence that Latvia actually ordered a boat from Thornycroft, although in many (secondary) sources this is suggested.
The boat ordered by Estonia (contracted 24 January 1939, yard number 2422) was taken over by Eire in September 1939.
Because it was completed earlier than the boats from the initial order, it became M1.
The initially ordered boats became M2 and M3.
Boats M4 to M6 were ordered in October 1939 and had yard numbers 2492-2494.
These had apparently no priority because they were completed and delivered very late.
The limited number of available photographs clearly show 3 types of Irish MTBs with M1 differing from M2 and M3.
M4-M6 had a different superstructure much like that of the British MTB 49.

Jan