As many saints of his time, Brendan the Navigator (c. 480 - c. 580) is both an historical and a mythical figure.
History describes him as an Irish monk, who founded churches and monastic communities across a freshly christianized Ireland. Mythology relates the wonderful Voyage of Saint Brendan, who sailed for years in search of Paradise - and eventually found it, after many adventures and tribulations. The region he had reached was later identified with the Canaries Island, then with the Azores, then with America.
The Voyage of Saint Brendan is a lovely piece of religious literature, probably based on real facts, then embelished by popular imagination. However, several sources, both from Norse authors and from Indian tribes, led a few historians to consider the idea of a Celtic settlement in America as plausible. See Leif Eriksson and Christopher Columbus, and wonder at the mystery of a continent that has been "discovered" so many times over history.