If your ancestor was an Irish born member of the British Merchant Marine from September 1918 to December 1921, you are in genealogical luck. The IRISH MARINERS web site has an online index of 23,000 Irish born seamen, with some Canadian seamen as well. The index is the key to a collection of cards in the Southampton Civic Archives. The cards themselves contain photos, personal information, and voyage details. The Irish Mariners site is fascinating to visit even if you do not have mariners of that time in your tree. Some of the photos of the young Irish men about to go off to sea are precious. Link to the website below.
If your family has tales of navy men among your ancestors, be sure to check the British naval records in the National Archives (U.K.). The Archives web page has a fantastic search engine, plus more records are being digitized and placed online as time goes on, so don't forget to check it often. Link to Archives' Naval Service records below.
I want to send a big THANK YOU to a few fellow researchers who alerted me to the Irish Mariners site: Jack Langton of the Castlecomer mailing list, Pat Connors of Connors Genealogy, and Michael of the County Carlow IGP site and Carlow mailing list. I would like to mention the hard work of the many independent site owners such as Pat and Michael, plus the tireless volunteers who maintain the Irish Genealogical Project pages. These individuals give their time and effort to bring free online records to Irish researchers everywhere. Links to their sites below.
IRISH MARINERS WEBSITE: http://www.irishmariners.ie/index.php
NATIONAL ARCHIVES NAVAL SERVICE RECORDS: http://www.nationalarchives.gov.uk/documentsonline/navy-cards-and-files.asp
COUNTY CARLOW IGP WEBSITE: http://www.rootsweb.ancestry.com/~irlcar2/Site_index.htm
CONNORS GENEALOGY (various Irish counties and NY records): http://www.connorsgenealogy.com/
BRENNAN WEBSITE (southeast Co. Laois): http://freepages.family.rootsweb.ancestry.com/~mjbrennan/index.htm