Rannoch, it wasn't the stitching that made the boat waterproof so much as it was the lanolin the hides were dressed with. I looked it up, out of curiosity: http://www.leathersmithe.com/the-brendan-voyage-select.html
The boat was made from 57 oak-tanned ox hides, tanned using traditional methods that take nearly a year to complete!
The thread they used was hand-spun flax, treated with black wax. Hides were overlapped by an inch or more, and double stitched for strength. One of Ireland's top harness makers oversaw the stitching, and taught them how to do both the traditional saddle stitch and a back stitch. He was a hard taskmaster, and would ruthlessly rip out someone's whole day's work if it wasn't up to his standards. Because of the size of the hides, the saddle stitch required 2 people, one working inside the hull of the boat and the other outside!
Strongly recommend the above link - it's not a long read, and it's FASCINATING!