Be careful when referring to St Patrick in any historical context as a lot of what we know about him was written by none other than himself (see link). As a historian you must question the sources of information and obviously there is the question of accuracy when it comes to his writings - did he write them, is what he wrote true etc?
St. Patrick is usually credited as being a major source of Christianity in Ireland. Although he was not the first Christian in Ireland -- his letters mention that a few Christians lived there before his 5th century mission -- he still was responsible for the proliferation and organization of Irish Christianity, and the adaptations to local culture that led to the creation of a distinctly Celtic or Ionan form of Christianity that persisted until the seventh century Synod of Whitby harmonized the distinctively Celtic features of Irish and Northumbrian Christianity with the more common Roman practices.
Saint patrick was kidnapped by slave traders and taken to Ireland when he was about 16. He stayed there for 6 years and then escaped. He came back a few years later and ministered in what is modern-day Northern Ireland from AD 428 onwards.