Saint Patrick was a 5th-century Romano-British Christian missionary and Bishop in Ireland. Much of what is known about Saint Patrick comes from the Declaration, which was allegedly written by Patrick himself. It is believed that he was born in Roman Britain in the fourth century, into a wealthy Romano-British family.
According to tradition, Patrick returned to Ireland to convert the pagan Irish to Christianity. The Declaration says that he spent many years evangelising in the northern half of Ireland and converted thousands. Patrick’s efforts were eventually turned into an allegory in which he drove “snakes” out of Ireland, despite the fact that snakes were not known to inhabit the region.
Saint Patrick’s Day, or the Feast of Saint Patrick, is a cultural and religious celebration held on the traditional death date of Saint Patrick (c. 385 – c. 461), the foremost patron saint of Ireland. The day commemorates Saint Patrick and the arrival of Christianity in Ireland, and celebrates the heritage and culture of the Irish in general.
Sources:
https://en.wikipedia.org/wiki/Saint_Patrick%27s_Day