Southampton Archives Services
They hold a number of card indexes for merchant seamen covering the period 1918 to 1941 and containing in total over 1,250,000 cards.
You should contact them to order copies of the original cards.
The Miramar Ship Index
This index compiled by Rodger Haworth, enables the ship to be identified from its official number. Eg ON 124093 is the Cunarder Mauretania
CLIP Crew List Index Project
This project aims to improve access to the records of British merchant seafarers for the period 1861 to 1913 and has developed a large database of crew list entries.
Mariners-L website and mailing list
A very useful website and mailing list for discussing and researching the history of merchant mariners.
Maritime History Archive. Memorial University of Newfoundland
They have an online index for crew agreements, which uses the official number (ON) of the ship. They possess many of the agreements for this period themselves but they also detail what other archives hold.
Be your own researcher
I have found the following book to be very useful in explaining the Who?, What?, Why?, When? and Where? questions on the British merchant marine system and the men who worked within it. It includes sailors from Ireland
'My Ancestor was a Merchant Seaman' by C T and M J Watts.
Published by the Society of Genealogists. ISBN 190 346 2517
Can be ordered from their website on www.sog.org.uk
There are errors and omissions on the database
Let me know the details. Impersonation is not unknown. Relations are great for validation. One relation informed me that the man shown on the card had given his brother's details.