The Story
Saint Patrick was a Roman Catholic Bishop in Ireland. As a child he performed many miracles, and later drove all the snakes off the island. Later, using shamrocks as an analogy to teach about the Trinity, he converted all of Ireland to Roman Catholicism.
As patron saint of Ireland, he protects the Irish people, who venerate him and pray to him for help and guidance.
Facts
Christianity
The Celtic Church in Ireland had been founded by the original Apostles during the first century. For centuries it followed the original teachings of Jesus and those Apostles, honouring the seventh day of the week as the Sabbath, celebrating the biblical festivals and holy days, baptizing adults using full immersion, and abstaining from unclean meat.
These Irish Christians would have totally rejected the new teachings and doctrines added by Rome. Unacceptable to their Christian way of life were pagan beliefs and practices such as the immortality of souls, an afterlife of bliss in Heaven or torture in Hell, the Trinity, transubstantiation, the veneration of saints, the worship of Mary, and the use of statues, relics, and other forms of idolatry. They would never participate in such non-biblical holidays and events as Easter, Christmas, Lent, or Hallowe'en. The idea of the primacy of Peter and the Pope would have been unacceptable; the Bible was the only authority of truth.
It was near the end of this period that Patrick became an important figure in Ireland, though certainly not as a Roman Catholic Bishop.
Roman Catholicism
In 596, a century after Patrick's death, Pope Gregory sent emissaries to the British Isles in order to convert the heretics
there to the Roman version of Christianity.
They were successful in central England, but to the west in Wales and Ireland they had more difficulty.
The Celts believed themselves to be Christian already, and that it was actually the Romans that were the heretics.
There is no record of any involvement of Patrick with the Roman church, and no record at all of Patrick himself for several centuries after his death. It wasn't until the late 800s that his fantastic myths and legends began to appear.
The Roman Church eventually succeeded, just as they had throughout Europe, using the same technique they had used to convert
many other cultures.
They named Patrick a saint, declared him the protector of Ireland, and created a mythology about this Irish folk hero to recruit followers.
Saying that Patrick rid Ireland of snakes and that Patrick had used the Irish shamrock to illustrate the Trinity made their teachings all the more believable.
Of course there hadn't been any snakes in Ireland since before the ice age, and Patrick himself would have rejected the Trinity doctrine. Patrick wasn't even Irish; he originally came from Scotland and was taken to Ireland as a slave.
Why
The Roman Church has always spread itself by incorporating existing religious beliefs and cultural practices into its teachings. Generally, the people were allowed to continue as they always had, with their old practices being retained with slight modifications and renaming to make them sound more like the Roman version of Christianity. Replacing local gods by patron saints, often by doing nothing more than changing their names, the Church has been able to convert millions.
By replacing Passover with Easter and declaring March 17 as a holy day dedicated to Saint Patrick, the Enlightener of Ireland
, the Roman Church soon converted Ireland.
Even today, when Saint Patrick's Day falls within Lent, the usual restrictions on food, drink, celebration, etc. that apply to the rest of the Catholic world are suspended in Ireland, making Saint Patrick even more beloved by the Irish.