The backstory to St Patrick's life and times is a story in itself. Merchants from various Mediteranean countries had been trading with Britain for quite a while, mostly tin from the southwest peninsula. Julius Caesar landed and explored there from Gaul in 56-54 BC, but didn't establish a presence there. The actual permanent conquest was in 43AD, but there was resistance and insurrection until the 60's. The common imperial practice was for several regiments of legionnaires to be stationed in the provinces, as well as a governor and some high-level bureaucracy; the lower administration was recruited from the local areas. By the time Patrick (Patricius) was born in Roman Britain in 385-393AD, this was an established Roman province. His father was a civil servant of the Roman imperial administration; his grandfather was a priest, possibly of a good position .The sons of the local leaders were educated in Latin and we see from Patrick's writings that he was familiar enough with the faith to call on it in need.
There were Christians in Britain very early; there are legends that Joseph of Arimathea came to Britain in about 65 AD; there was trade and contact with Gaul as well as Rome through the first century, and Rome sent missionaries in 167AD. There are records of churches that are amazingly extensive, but there was still controversy--the first recorded martyr was St Alban in 209AD.
Britain served as a major provincial political stepping stone through this period; Constantine was the Legion commander in Britain in 305; when the word of his father, Emperor Constantius, was received, the troops in Britain pledged support. He had to return and deal with other claimants to the throne; as he arrived near Rome in 325, he had a vision of a cross and the words "In this sign, conquer." He decreed the end of persecution and discrimination against the church, and by the end of the century, the empire was officially Christian, so Patrick was born and raised in a stable, Christian society. As he was growing up, however, the Legions had to be recalled to Rome to deal with invasions of Huns and other barbarians. By 410, the Romans were gone, and the Britons on their own. During the time Patrick was in Ireland, Britain was described as facing civil wars, plagues, heresy, which probably explains the resurgence of British slave raids on Ireland. Not much later, there were incursions and invasions of pagan tribes from the continent.
Patrick founded communities that taught and discipled his followers; he trained men who moved out to Scotland, back to Britain, and to the continent, who preached the Gospel and saved and copied libraries that were in danger of destruction by the invasions sweeping across Europe. God had prepared the ground for the emergence of a remarkable man who not only changed Ireland, but preserved His Word in Britain and brought it to Scotland as well as a good part of Europe. We see an example here is God's providence--He doesn't insulate us from trouble, but He takes care of His people.