St. Patrick, Apostle of Ireland, was sent to that country by Pope St. Celestine as a missionary. He found Ireland heathen and left it Christian. St. Patrick died A.D. 464, and was buried at Down in Ulster. He scattered the seed of the Gospel with such success that from the innumerable band of holy men and women which it produced, the verdant land of Erin was known in the Middle Ages by the glorious title of the “Island of Saints” – a glory which three centuries of bitter persecution of the Catholic Faith at the hands of the Anglican Church utterly failed to eclipse, Pius IX in 1859 as a tribute to the vigorous faith of this nation raised the feast of St. Patrick which has appeared in the Roman Breviary since the Fifteenth Century, to the rank of a double. Patrick is the great patriarch of the Irish episcopate, and of Irish monachism. This monachism left its mark throughout mediaval Europe wherever the Scotti planted their tents and introduced their traditions. His feast is a holy day of obligation in Ireland; there is a church dedicated to him in Rome, not far from the Via Salaria.
(Source: Fr. Lasance, The New Roman Missal)