The ever erudite Cnytr has an excellent post commemorating the day.
With all the leprechauns and green beer it's easy to miss the significance of Patrick's missionary work. Firstly, thanks to his labours and those of his companions the warlike Irish were converted to Christianity in the space of less than two generations without any martyrdoms. Secondly, Patrick represents the Church re-discovering its missionary nature. After the Apostolic era and early Patristic era and the mission to the gentiles there wasn't a huge amount of strictly missionary activity within the church. Christianity seemed to spread 'naturally' within the Empire and this situation was exacerbated by the conversion of Constantine and the consequent elevation of Christianity to the level of an official empire. It was therefore quite a bold step of Patrick to move beyond the bounds of the 'safe' world of the Empire to the barbaric island of Hibernia where he had already been a slave. Indeed, his missionary efforts seem to have been opposed by the bishops of Britain who did not seem to recognise the value of bringing the Gospel to these barbarians.
The fact that the conversion to Christianity so unbloody inspired many an Irish monk to exile himself from his homeland as a substitute for the martyrdom otherwise denied him and it was this that contributed so much to the presevation and restoration of learning after the dark ages.
Patrick wrote of his mission in Ireland as follows:
Now, it would be tedious to give a detailed account of all my labours or even a part of them. Let me tell you briefly how the merciful God often freed me from slavery and from twelve dangers in which my life was at stake, not to mention numerous plots, which I cannot express in words; for I do not want to bore my readers. But God is my witness, who knows all things even before they come to pass, as He used to forewarn even me, poor wretch that I am, of many things by a divine message.
How came I by this wisdom, which was not in me, who neither knew the number of my days nor knew what God was? Whence was given to me afterwards the gift so great, so salutary, to know God and to love Him, although at the price of leaving my country and my parents?
And many gifts were offered to me in sorrow and tears, and I offended the donors, much against the wishes of some of my seniors; but, guided by God, in no way did I agree with them or acquiesce. It was not grace of my own, but God, who is strong in me and resists them all, as He had done when I came to the people of Ireland to preach the Gospel, and to suffer insult from the unbelievers, hearing the reproach of my going abroad, and many persecutions even unto bonds, and to give my free birth for the benefit of others; and, should I be worthy, I am prepared to give even my life without hesitation and most gladly for His name, and it is there that I wish to spend it until I die, if the Lord would grant it to me.
For I am very much God's debtor, who gave me such grace that many people were reborn in God through me and afterwards confirmed, and that clerics were ordained for them everywhere, for a people just coming to the faith, whom the Lord took from the utmost parts of the earth, as He once had promised through His prophets: To Thee the gentiles shall come from the ends of the earth and shall say: `How false are the idols that our fathers got for themselves, and there is no profit in them'; and again: `I have set Thee as a light among the gentiles, that Thou mayest be for salvation unto the utmost part of the earth.'
And there I wish to wait for His promise who surely never deceives, as He promises in the Gospel: They shall come from the east and the west, and shall sit down with Abraham and Isaac and Jacob, as we believe the faithful will come from all the world.