Many of the stories around St. Patrick are legends that accumulated over the centuries. There are, however, some documents that scholars generally accept as having likely been written by Patrick. One is an autobiography written in 450, 16 years before his death: http://www.cin.org/patrick.html
Born in England, he was captured and taken to Ireland as a slave when he was 16. He worked in the fields and never received a formal education. At 22 he escaped. Eventually he returned as an evangelist. He died a bishop, having baptized thousands.