Saint Patrick lived during the period of 400 and 500 a.d.. He was born in England, but was captured and taken as a slave to Ireland where he lived for years, eventually escaping and returning to England. After he had returned to England, he had a vision of the people of Ireland calling him to return.
He returned to Ireland and became a part of the christian church, converting people from their pagan beliefs. There are many stories about Saint Patrick, mostly myths. For example, the most famous is that he drove the snakes from Ireland. However, there is no evidence that there were any snakes in Ireland at the time he lived. Some believe that the word "snakes" was an indirect reference to the pagans. Saint Patrick was also accused of financial impropriety -- taking money from converts who wanted to be "saved" -- so did not live a life without controversy.
Perhaps the biggest controversy is whether the stories told about him might actually be a combination of stories about two different men. This is called the Two Patricks theory.
At some point in his life in Ireland, Saint Patrick had some doubts about his beliefs, and he went to a lake, and an island, and prayed. The place where Saint Patrick went is called Lough (Lake) Derg. People to this day go there and spend some time in solitude, in prayer or meditation, for spiritual renewal.
Over time, Saint Patrick became known as the patron saint of Ireland, and March 17 (supposedly the day of his death) is now known around the world as a day on which to celebrate all things Irish.