Ancestry has a new resource that might be useful to those with Irish ancestors who took to the waters. It's the UK Royal Navy Registers of Seamen's Services 1900 to 1928 collection.
The records do exactly what you'd expect from the tin: they record the service of seamen in the Royal Navy, so you can discover which vessels they served on and the dates of service. They also include birthplace, prior occupation, physical description, reasons for discharge and other personal details. (I've found one of my Santrys recorded as 'invalided' by reason of gonorrhea... ah, the stuff of family history, just made for dinner-table discussion.)
The title of the record set could be a little misleading. These registers include seamen who began their service between 1900 and 1918; their dates of service may continue up to 1928.
As far as I can tell, there are a total of nearly 400,000 seamen included in this collection, which is held by the National Archives of the UK in Kew, London. Around 6,300 of the records relate to men born in Ireland.
Just a word of warning. The geographical awareness of the indexers or OCR equipment that prepared this database is pretty poor. With very little effort, I've found Skibbereen in County Cork recorded as Shibbereen, Sabbereen, Kibbereen, Skibbecen, Skibberun and Shibberan; Clonakilty as Cloakelty, Clona Kilty and Elonakilty; Drimoleague as Dkimoleagus; and Roscarferry, which at least is funny.
Outside of Cork there's Ballb??ggan, Lectrim, Bray in Wecklow, and Kathmines and ??ords in Dublin. There are also at least 10 men recorded as signing up aged in their 50s and 60s, which I don't quite believe. In other words, be a bit imaginative when you search this collection.