Writings about St Patrick abound. You can see some of the legends here. If you want to see some of the earliest writings, you can consult: The tripartite life of Patrick : with other documents relating to that Saint (2 vols, 1887, part of the magnificent Rolls Series), and Lives of saints, from the Book of Lismore (1890). For those who want to see the original manuscripts upon which the above works are based, you can come here to view Irish manuscripts from various institutions.
Life and Legends of St Patrick