St Patrick was a real person. He was born in Britain (probably in Wales) in 5th century AD. His father, Calpurnius, was a Roman official. St Patrick was originally a pagan, not a Christian. He was taken to Ireland as a slave when he was 16 years old. He escaped after 6 years and became a Christian priest. In due course he became a Bishop and returned to Ireland as a missionary. He played a major part in converting the Irish to Christianity. Some of his writings survive, the Confessio and the Letter to Coroticus.
Saint Patrick is the patron saint of Ireland. St Patrick's Day is March 17. St Patrick is traditionally associated with the Shamrock plant, which he used to explain the concept of the Trinity.