Patrick, Saint, the Apostle of Ireland, was of Scotch birth. His proper name was Succathus; the name by which we designate him is of Latin origin; patricius means noble, illustrious; it was a surname and a title of honor at the same time given to him by his grateful admirers. Patrick was wild and wicked until his sixteenth year, when he remembered the God of his fathers and repented him of his sins, and enlisted in the divine service. There is no ground for doubting but that he preached the gospel of repentance and faith in Ireland, and that his ministrations were attended by overwhelming success. There are accounts extant of a number of his baptisms, but they are all immersions. There is one baptism mentioned by Nennius (History of the Britons, p. 410. Bohn, London) and by Todd (St. Patrick, Apostle of Ireland, p. 449. Dublin), and found in many other histories, of which O’Farrell writes (Popular Life of St. Patrick, p. 110. New York, 1863), “When the saint entered Tirawly the seven sons (of Amalgaidh) assembled with their followers. Profiting by the presence of so vast a multitude, the apostle entered into the midst of them, his soul inflamed with the love of God, and with a celestial courage preached to them the truths of Christianity; and so powerful was the effect of his burning words that the seven princes and over twelve thousand more were converted on that day, and were soon after baptized in a well (a spring or fountain) called Tobar Enadhaire, the well of Enadhaire.” A number of other fountain baptisms of St. Patrick may be found in “The Baptism of the Ages,” pp. 62-70. Publication Society, Philadelphia. We have strong reasons for regarding St. Patrick as a Baptist missionary, and beyond contradiction his baptism was immersion.
– The Baptist Encyclopedia: Edited by William Cathcart. (1883) pp. 886-7
Also, let me highly recommend reading, in his own words, The Confession of Saint Patrick.