A website with an online index of Irish born merchant seamen (and one woman) contained in the CR10 series of index cards in the Southampton Civic Archives. The website currently contains information, including date of birth, on 11,345 Irish born merchant seamen active from late 1918 to the end of 1921 out of an estimated total of around 20,000 plus. You need to know both first and surname, or identity number, to search.
The Southamption Civic Archives holds 270,000 cards covering the the British Merchant Marine during the period from late 1918 to the end of 1921 which usually includes passport-type photographs. It is part of the Central Index Register of Merchant Seamen, a collection of over a million and a quarter merchant navy service record cards with details of people serving on board British registered ships between 1918 and 1941.