Severin needed a genuine mediaeval Irish craft to replicate the journey so that he could prove that an Irish boat could have reached America. The curragh was a larger version of the modern Irish boat, constructed according to mediaeval drawings. It was made of oxhide leather stretched over an ash frame, the leather was tanned in sheep grease, the traditional Irish waterproofing.
Unlike the small modern curragh, it had masts, two of them, though there were also oars. Severin later realized that he was underpowered with oarsmen and that Brendan would have needed a crew larger than Severin had.
Routes to America
There are two sea routes from Europe to North America for sailing boats: the Caravel Route taken by Columbus from Spain to the Caribbean, and the Stepping Stone Route, which arcs round from Kerry through the North Atlantic islands to end at Newfoundland. When Severin, an expert in ancient navigation, examined the Navigatio, he realized that its author knew the Stepping Stone Route, and that this indicated that the tale was based on a real journey, as the route could only be known by one who had done it successfully and returned.
The route that Severin took was what he thought Brendan would have taken. It ran from Brandon creek in Kerry (where Brendan was said to have lived), up along the coast, skirting out into deeper ocean to avoid the perilous, precipitous cliffs of Moher, to the safety of the Inish Mor in the Aran islands, where Brendan would have stayed at the monasteries there.
From the Aran islands, it goes along the Irish coast followed by a run through the Hebrides, stopping in various places. An important stop was the sacred isle of Iona, where Columcille [Columba] had established a monastery that would welcome the voyagers, Brendan then went on to Lewis, before reaching open ocean.