But as time wore on, the Navigatio -- along with St. Brendan himself -- passed into the realm of legend. If Brendan had lived -- as most scholars assume -- surely he couldn’t have traveled across the treacherous North Atlantic with the technology available at the time. Certainly, he couldn’t have beaten the Vikings to North America.
Ironically, it is Viking lore that lends support to the idea that Brendan was the first European in North America. Read the next page to find out about evidence for and against this idea.
St. Brendan -- or Somebody

One of the biggest problems with the idea that St. Brendan and his crew were the first Europeans to arrive in North America is the dearth of physical evidence to support this claim. Unlike the Vikings, there is no settlement that proves the Irish were here prior to other Europeans. At one time, however, tantalizing physical evidence did emerge.
http://people.howstuffworks.com/irish-monk-america1.htm