St Patrick is recognized as a saint of various Christian denominations, including the Catholic and Orthodox Churches.
It is generally accepted that he died on March 17 493, but that actual date and year is open to debate. Even the year of his birth is not known with any surety.
Patrick was born in a village that he identified as Bannavem Taberniae, probably near the sea in southwestern Britain. Evidence does not allow a more exact date for his birth than sometime between 388 and 408. - St. Patrick
St Patrick's death probably occurred in 493, but some sources say otherwise:
Apostle of Ireland, born at Kilpatrick, near Dumbarton, in Scotland, in the year 387; died at Saul, Downpatrick, Ireland, 17 March, 493. Some sources say 460 or 461. --Ed. - Catholic Encyclopedia
The only two sources that are recognized to have been written by St Patrick are his "Confessio" and the "Epistola ad Coroticum". Any other writings about St Patrick or written by St patrick are of dubious origin or date.
The "Confessio" and the "Epistola ad Coroticum" are recognized by all modern critical writers as of unquestionable genuineness. The best edition, with text, translation, and critical notes, is by Rev. Dr. White for the Royal Irish Academy, in 1905. The 34 canons of a synod held before the year 460 by St. Patrick, Auxilius, and Isserninus, though rejected by Todd and Haddan, have been placed by Professor Bury beyond the reach of controversy. Another series of 31 ecclesiastical canons entitled "Synodus secunda Patritii", though unquestionably of Irish origin and dating before the close of the seventh century, is generally considered to be of a later date than St. Patrick. Two tracts (in P.L., LIII), entitled "De abusionibus saeculi", and "De Tribus habitaculis", were composed by St. Patrick in Irish and translated into Latin at a later period. Passages from them are assigned to St. Patrick in the "Collectio Hibernensis Canonum", which is of unquestionable authority and dates from the year 700 (Wasserschleben, 2nd ed., 1885). This "Collectio Hibernensis" also assigns to St. Patrick the famous synodical decree: "Si quae quaestiones in hac insula oriantur, ad Sedem Apostolicam referantur." (If any difficulties arise in this island, let them be referred to the Apostolic See). The beautiful prayer, known as "Faeth Fiada", or the "Lorica of St. Patrick" (St. Patrick's Breast-Plate), first edited by Petrie in his "History of Tara", is now universally accepted as genuine. The "Dicta Sancti Patritii", or brief sayings of the saint, preserved in the "Book of Armagh", are accurately edited by Fr. Hogan, S.J., in "Documenta de S. Patritio" (Brussels, 1884). The old Irish text of "The Rule of Patrick" has been edited by O'Keeffe, and a translation by Archbishop Healy in the appendix to his Life of St. Patrick (Dublin, 1905). It is a tract of venerable antiquity, and embodies the teaching of the saint. - Writings of St. Patrick
The Hiberno-Roman relations during the fifth century are explained here:
The relationship between Rome and Hibernia was mostly commercial. Scholar Richard Warner in 1995 wrote that after emperor Claudius invasion of southern Britannia, the trade routes between the Mediterranean sea and Roman Britannia encompassed even Hibernia. The geographer Ptolemy, in his map of the 1st century AD pinpointed the coastal settlements and tribes of Ireland, showing a knowledge that (it is suggested) only merchants could have achieved in that century. Additionally, there are many Roman archaeological objects (mainly jewellery and Roman coins) found in areas of central and southern Ireland (such as Tara and Cashel), that reveal a relationship. Roman coins have also been found at Newgrange.
According to the theory of Thomas Charles-Edwards, who wrote about the Irish Dark Age, between the 1st and 3rd century there was a depopulating slave trade from Hibernia toward rich Roman Britain, that had an economy based on villa farming and needed slaves to perform the heaviest labour in agriculture. - Hiberno-Roman relations
Whatever the state of commerce was between Ireland and the rest of Europe, we must remember that Fall of Rome happened in 476. Ireland had never been conquered by the Romans.
As the political boundaries of the Roman Empire diminished and then collapsed in the West, Christianity spread beyond the old borders of the Empire and into lands that had never been under Rome.
Irish and Irish missionaries
Beginning in the fifth century, a unique culture developed around the Irish Sea consisting of what today would be called Wales and Ireland. In this environment, Christianity spread from Roman Britain to Ireland, especially aided by the missionary activity of St. Patrick with his first-order of 'patrician clergy', active missionary priests acompanying or following him, typically Britons or Irish ordained by him and his successors. Patrick had been captured into slavery in Ireland and, following his escape and later consecration as bishop, he returned to the isle that had enslaved him so that he could bring them the Gospel. Soon, Irish missionaries such as Columba and Columbanus spread this Christianity, with its distinctively Irish features, to Scotland and the Continent. One such feature was the system of private penitence, which replaced the former practice of penance as a public rite. - History of Christianity during the Middle Ages.