Ancestry's new US Applications for Seamen's Protection Certificates, 1916-1940, contains more than 315,203 records of seamen issued with certificates at US ocean and Great Lakes ports by customs officials. Nearly 3,350 of the records relate to Irish-born men.
A typical application contains the seaman's name, age, birthdate and birthplace, a physical description and photo. Sometimes they also include the birthplaces and names of parents, and place of naturalisation, if applicable.
While this collection is of obvious interest to anyone with Irish ancestry and maritime connections, I have to have a moan. In the first three pages of search results (ie the first 150 records), I found the following placenames listed as the birthplace of Irish-born sailors: Rey Kjavik, Dullin, Wisklow, Benbridge, Newey, Clenmel, Neptune, Maye, County Loud and Omeath. I think Ancestry needs to find someone who knows Ireland to spend a bit of time going through the transcriptions for this collection!