There is an entire genre of early medieval Irish literature devoted to describing amazing sea voyages. These mostly date from the 8th and 9th centuries, but the most well-known is the 10th century Navigatio Sancti Brendani Abbatis (Voyage of Saint Brendan the Abbot). In this tale, Brendan and 16 companions travel through a series of fantastic islands. Some have speculated that the story incorporates knowledge of a real voyage across the North Atlantic.

Here is a description of the ship from Siân Echard’s translation:

“Then Brendan and his companions, using iron tools, prepared a light vessel, with wicker sides and ribs, such as is usually made in that country, and covered it with cow-hide, tanned in oak bark, tarring its joints. They put provisions for forty days on board, along with enough fat to treat the hides covering the boat, and all the tools and utensils they needed.”

The ship has both sails and oars.

In 1976 a man named Tim Severin spearheaded a project to build a boat that combined this description with the skills of traditional currach builders in 20th century Ireland. The resulting 36 foot long vessel had two masts. Severin and a crew of three others sailed in from Kerry in Ireland to the Faroe Islands to Iceland to Greenland to Labrador to Newfoundland, Canada. This demonstrated that it was at least possible to make long voyages in a hide boat.

This is all much later than your period of late antiquity but there was a golden model of a boat found in Broighter, Northern Ireland, as part of a hoard of gold objects dated to the 1st century BCE. This boat had, originally, nine benches for rowers and 18 oars, a mast, a steering oar, a grappling iron and three forked instruments. One of the benches and three of the oars were broken off and lost. For comparison, Viking longships typically had 24 to 50 oars. This find suggests boats similar to Brendan’s already existed in this earlier period.