The original texts relating to the voyage are the "Navigatio Sancto Brendani Abbatis" circa 800AD. From these Tim Severin calculated a voyage was actually possible and set out to do a voyage in a boat reckoned to be available at the time.

Now it could be that the people who wrote the Navigatio had been eating magic mushrooms. We will never know unless or until artefacts are found on the North American continent.

I just happen to like the story so am more inclined to want it to be true.

But do read Tim Severins book it is fascinating.