Simple hide covered boats, durable enough for long distance travel but poor for combat.
The lightest of Celtic ships are of simple construction. They are round, with leather hides stretched over a simple wood frame, and covered in pitch to ensure they're water tight. A single boat of this type can only carry a few men, but they're very light and easy to build. In a pinch, many dozens of them could be made quickly for use as transports. Such boats could also be ferried aboard larger ships, and carried over land and used in rivers. Such transportation remained in use for centuries, culminating in the even smaller personal boats called 'Curragh/Curach' in the dark ages. These later boats incorporated a 'clam' shape so they could be closed during a storm. Early variants sometimes incorporated this feature, but were larger and could accommodate a small 'crew', rather than only one or two men. During battle, these small boats would likely paddle up next to a larger ship, and an attempt to scale the sides could be made from there. However, it would be infinitely wiser to avoid combat when employing such ships, as a single tear could sink one. Due to their construction and small size, they are best used for brief hops when transporting an army, to avoid their destruction at the hands of a superior fleet.