I don't doubt the seaworthiness of the curragh. I recently watch some lovely footage of the Arran Islanders beach-launching a curragh into a big sea.
I'm just looking at the evidence available. We know for a fact that folk in the Bronze Age were using large wooden hull boats. I just wonder when that began. I think it's resonable to assume that the first large sea-going vessels were probably skin-hulled boats but as we know from the monuments, the people of the Neolithic didn't always take the simplest option.

I guess the Inuits use of the Umiak was borne out of neccessity, there's bugger all wood up in the high arctic.