Number One Son had a party at the back of the house last night and I did the middle aged Dad thing and watched the telly in the front room. The best thing on, apart from Life of Brian of course, was an episode of the excellent BBC/Open University series Coast, in which the amiable Neil Oliver learned to row a currach on the west coast of Ireland. As he had just talked to surfers about the truly terrifying waves that crash into that rocky coast, this was actually rather a courageous thing to do. And by the magic of BBC iPlayer, you can view the whole thing on your computer for the next seven days, here.
To cope with the swell, curraghs use thin oars with barely any blade at all so catching to tops of waves on the return stroke will not cause problems. I think they grip the water because of the length. The oars swivel on thole pins rather than oarlocks so to keep the correct angle to the water.
Curraghs have an amazing seakeeping ability partly because of their flexible canvas-on-frame construction. There was a super example for sale at the Thames Beale Park Boat Show a couple of years back. Coracle maker supreme Peter Faulkner was selling it - you can see him building a currach out of twigs and cowhide here.