The classic and most well-known vessel of the "Celts" of the British isles is the coracle (in Britain) or currach (in Ireland and Scotland) Coracles IIRC are mostly of woven wood pieces, circular, and fairly smaller, mostly for rivers etc. while currach are larger, made of hides over a wooden frame and are not necessarily circular. I believe there are also references in Dark Age Irish texts to larger vessels suited for longer voyages. Norse longships were also requisitioned and copied from around the 10th Century.

The most interesting aspect of Celtic seafaring I can think of, though I don't know much about it myself, are the ships of the Veneti, the Gallic tribe from Armorica/Britanny whose fleet is described by Caesar as being composed of very large (to Mediterranean eyes) tall ships powered solely by sails, which strikes me as quite unusual for the classical era. Maybe someone with a copy of De Bello Gallico handy can provide a reference.

Antagonist