What these were made of is uncertain, however it's quite possible that they used large logs made from oaks or pine that were doubled on top of one another where they were tied down with soft springy saplings that were split lengthwise and bent over the logs horizontaly to the top and the underside.
These springy saplings with pliability were then tied at the ends together with leather similar to how a gripper bar raft is made.
It is quite possible that they may have brought pigs and goats because they could survive with little care in the coastal forests and they seem to do well together since pigs eat roots and tubers while goats can eat small twigs, branches and lichens.
The west coasts of Ireland and Scotland which presently have moors would have looked like forests since the warm gulf stream would not have had a full effect at this stage.
The people sent there over the centuries came from either the Bay of Biscay or from the western coasts of Ireland which they may have used as a repair and resting staion.