Saint Brendan is primarily known for the seven year voyage he took into the Atlantic with 60 followers. According to a 9th century manuscript The Voyage of Saint Brendan the Navigator he set off to search for the Garden of Eden. This probably occurred around 530 AD, before he traveled from Ireland to the island of Britain.
On his journey Brendan is said to have seen an island covered in vegetation, which he believed to be Paradise. Some claim that he may have discovered North America in his travels. He also encountered a sea monster (probably a whale) Christopher Columbus used the legends told of St Brendan as part of his argument that it was possible to travel to Asia by crossing the Atlantic.
Later, he traveled to Wales and the holy island of Iona, off the west coast of Scotland. Upon his return to Ireland, he founded a bishopric at Annaghdown, where he spent the rest of his days. He was recognized as a saint by the Irish church, and his feast day is May 16.
Saint Brendan also is one of the few saints to be featured as a comic book character. In Dr. Strange III #11/2 (December 1989) he was summoned by Merlin and it took their combined power to contain the Darkhold and trap Mordred in a special crypt. More about Saint Brendan the Marvel comic book character can be found Here.