St. Patrick c. 387 – 17 March c. 460, the patron saint of Ireland one of Christianity's most widely known figures, was a Romano-British Christian missionary and bishop in Ireland. Known as the "Apostle of Ireland", he is the primary patron saint of the island along with Saints Brigid and Columba. But for all his celebrity, his life remains a mystery. Many of the stories traditionally associated with St. Patrick, including the famous account of his banishing all the snakes from Ireland, are false, the products of hundreds of years of exaggerated storytelling.

St. Patrick was born in Roman Britain at Banna Venta Berniae, a location otherwise unknown, though identified in one tradition as Glannoventa, modern Ravenglass in Cumbria. Calpornius, his father, was a deacon, his grandfather Potitus, a priest. Although his father was a Christian deacon, it has been suggested that he probably took on the role because of tax incentives and there is no evidence that Patrick came from a particularly religious family. When he was sixteen, Patrick was taken prisoner by a group of Irish raiders who were attacking his family's estate, he was captured and carried off as a slave to Ireland. There is some dispute over where this captivity took place. Although many believe he was taken to live in Mount Slemish in County Antrim, it is more likely that he was held in County Mayo near Killala. Patrick worked as a herdsman, remaining a captive for six years. Lonely and afraid, he turned to his religion for solace, becoming a devout Christian. (It is also believed that Patrick first began to dream of converting the Irish people to Christianity during his captivity.) He writes that his faith grew in captivity, and that he prayed daily.

After more than six years as a prisoner, Patrick escaped. According to his writing, a voice, which he believed to be God's, spoke to him in a dream, telling him it was time to leave Ireland. Fleeing his master, he travelled to a port, two hundred miles away, where he found a ship and, after various adventures, returned home to his family, now in his early twenties. After escaping to Britain, Patrick reported that he experienced a second revelation, an angel in a dream tells him to return to Ireland as a missionary. Patrick recounts that he had a vision a few years after returning home:

I saw a man coming, as it were from Ireland. His name was Victoricus, and he carried many letters, and he gave me one of them. I read the heading: "The Voice of the Irish". As I began the letter, I imagined in that moment that I heard the voice of those very people who were near the wood of Foclut, which is beside the western sea—and they cried out, as with one voice: "We appeal to you, holy servant boy, to come and walk among us."

Most available details of his life are from subsequent hagiographies, and these are now not accepted without detailed criticism. The Annals of Ulster state that he arrived in Ireland in 432, ministered in Ulster around 443, and died in 457 or 461. The text, however, distinguishes between "Old Patrick" and "Patrick, archapostle of the Scots," who died in 492. The actual dates of Patrick's life cannot be fixed with certainty but, on a widespread interpretation, he was active as a missionary in Ireland during the second half of the 5th century. He is generally credited with being the first bishop of Armagh, Primate of All Ireland.

Most modern scholars of Saint Patrick follow a variant of T. F. O'Rahilly's "Two Patricks" theory. That is to say, many of the traditions later attached to Saint Patrick actually concerned Palladius, who Prosper of Aquitaine's Chronicle says was sent by Pope Celestine I as the first bishop to Irish Christians in 431. Palladius was not the only early cleric in Ireland at this time. The Irish-born Saint Ciaran Saighir the Elder lived in the later fourth century (352–402 AD) and was the first bishop of Ossory. Ciaran the Elder along with Saints Auxilius, Secundinus and Iserninus are also associated with early churches in Munster and Leinster. By this reading, Palladius was active in Ireland until the 460s.

Familiar with the Irish language and culture, Patrick chose to incorporate traditional ritual into his lessons of Christianity instead of attempting to eradicate native Irish beliefs. For instance, he used bonfires to celebrate Easter since the Irish were used to honouring their gods with fire. He also superimposed a sun, a powerful Irish symbol, onto the Christian cross to create what is now called a Celtic cross, so that veneration of the symbol would seem more natural to the Irish. Although there were a small number of Christians on the island when Patrick arrived, most Irish practised a nature-based pagan religion. He writes that he "baptised thousands of people". He ordained priests to lead the new Christian communities. He converted wealthy women, some of whom became nuns in the face of family opposition. He also dealt with the sons of kings, converting them too. The Irish culture centered around a rich tradition of oral legend and myth. When this is considered, it is no surprise that the story of Patrick's life became exaggerated over the centuries, spinning exciting tales to remember history has always been a part of the Irish way of life.

Charges were made against St. Patrick by his fellow Christians at a trial. What these charges were, he does not say explicitly in his writings, but he writes that he returned the gifts which wealthy women gave him, did not accept payment for baptisms, nor for ordaining priests, and indeed paid for many gifts to kings and judges, and paid for the sons of chiefs to accompany him. It is concluded, therefore, that he was accused of some sort of financial impropriety, and perhaps of having obtained his bishopric in Ireland with personal gain in mind. St. Patrick's position as a foreigner in Ireland was not an easy one. His refusal to accept gifts from kings placed him outside the normal ties of kinship, fosterage and affinity. Legally he was without protection, and he says that he was on one occasion beaten, robbed of all he had, and put in chains, perhaps awaiting execution.

When he arrived in Armagh, somewhere towards the end of his mission to Ireland, Patrick wanted to build a great stone church on what was once a very important pagan hilltop site. This was the mound where Queen Maeve had ruled from before moving to nearby Navan Fort. At that time Ireland was largely woodlands and lakes and its people were more or less still hunter/gatherers. Virtually all buildings would have been wooden. Stone represented strength and permanence. His first request to build was refused. He had approached Dara the Chieftain of the area for permission to build his church on this hilltop. Dara, who was not at all happy with this new Christian practice, would not give permission but instead gave him an alternative site in Ferta Street in Armagh city(now Scotch Street) on which to build. This site was well away from the original hilltop he wanted but Patrick went ahead and built a small wooden hut there - people in Armagh call this his 'temporary residence' because, eventually, Dara embraced Christianity and did give Patrick the ancient hilltop site. Patrick built his great stone church there in 445AD and made Armagh the centre of the church in Ireland. The Church or Ireland Cathedral now sits on the very same spot. Armagh remains to this day the centre of the church and is now known as the Ecclesiastical Capital or Christian Capital of Ireland."

The absence of snakes in Ireland gave rise to the legend that they had all been banished by St. Patrick. chasing them into the sea after they attacked him during a 40-day fast he was undertaking on top of a hill. However, all evidence suggests that post-glacial Ireland never had snakes, as on insular "New Zealand, Iceland, Greenland and Antarctica. So far, no serpent has successfully migrated across the open ocean to a new terrestrial home" such as from Scotland at one point only eight miles from Ireland, where a few native species have lived, "the venomous adder, the grass snake, and the smooth snake", as National Geographic notes, and although sea snake species separately exist. "At no time has there ever been any suggestion of snakes in Ireland, so [there was] nothing for St. Patrick to banish", says naturalist Nigel Monaghan, keeper of natural history at the National Museum of Ireland in Dublin, who has searched extensively through Irish fossil collections and records. The List of reptiles of Ireland has only one land reptile species native to Ireland; the viviparous or common lizard.

St. Patrick's walking stick grows into a living tree. Some Irish legends involve the Oilliphéist, the Caoránach, and the Copóg Phádraig. During his evangelising journey back to Ireland from his parent's home at Birdoswald, he is understood to have carried with him an ash wood walking stick or staff. He thrust this stick into the ground wherever he was evangelising and at the place now known as Aspatria (ash of Patrick) the message of the dogma took so long to get through to the people there that the stick had taken root by the time he was ready to move on.

According to the latest reconstruction of the old Irish annals, Patrick died in AD 460 on March 17, a date accepted by some modern historians. Prior to the 1940s it was believed without doubt that he died in 420 and thus had lived in the first half of the fifth century. While Patrick's own writings contain no dates, they do contain information which can be used to date them. Patrick's quotations from the Acts of the Apostles follow the Vulgate, strongly suggesting that his ecclesiastical conversion did not take place before the early fifth century. Patrick also refers to the Franks as being pagans. Their conversion is dated to the period 496–508. There is plentiful evidence for a medieval tradition that Patrick had died in 493. An addition to the Annals of Ulster states that in the year 553. In the Book of Cuanu: The relics of Patrick were placed sixty years after his death in a shrine by Colum Cille. Three splendid halidoms were found in the burial-place: his goblet, the Angel's Gospel, and the Bell of the Testament. This is how the angel distributed the halidoms: the goblet to Dún, the Bell of the Testament to Ard Macha, and the Angel's Gospel to Colum Cille himself. The reason it is called the Angel's Gospel is that Colum Cille received it from the hand of the angel. The reputed burial place of St. Patrick in Downpatrick The placing of this event in the year 553 indicate a tradition that Patrick's death was 493, or at least in the early years of that decade, and the Annals of Ulster report under 493:

Patrick, arch-apostle, or archbishop and apostle of the Irish, rested on the 16th of the Kalends of April in the 120th year of his age, in the 60th year after he had come to Ireland to baptise the Irish.

The National Museum of Ireland in Dublin possesses a bell first mentioned, according to the Annals of Ulster, in the Book of Cuanu in the year 552. The bell was part of a collection of "relics of Patrick" removed from his tomb sixty years after his death by Colum Cille to be used as relics. The bell is described as "The Bell of the Testament", one of three relics of "precious minna" (extremely valuable items), of which the other two are described as Patrick's goblet and "The Angels Gospel". Colum Cille is described to have been under the direction of an "Angel" for whom he sent the goblet to Down, the bell to Armagh, and kept possession of the Angel's Gospel for himself. The name Angels Gospel is given to the book because it was supposed that Colum Cille received it from the angel's hand. A stir was caused in 1044 when two kings, in some dispute over the bell, went on spates of prisoner taking and cattle theft. The annals make one more apparent reference to the bell when chronicling a death, of 1356, "Solomon Ua Mellain, The Keeper of The Bell of the Testament, protector, rested in Christ." The bell itself is simple in design, hammered into shape with a small handle fixed to the top with rivets. Originally forged from iron, it has since been coated in bronze. The shrine is inscribed with three names, including King Domnall Ua Lochlainn's. The rear of the shrine, not intended to be seen, is decorated with crosses while the handle is decorated with, among other work, Celtic designs of birds. The bell is accredited with working a miracle in 1044 and having been coated in bronze to shield it from human eyes, for which it would be too holy. It measures 12.5 × 10 cm at the base, 12.8 × 4 cm at the shoulder, 16.5 cm from base to shoulder, 3.3 cm from shoulder to top of handle and weighs 1.7 kg.

St. Patrick features in many stories in the Irish oral tradition and there are many customs connected with his feast day. The folklorist Jenny Butler discusses how these traditions have been given new layers of meaning over time while also becoming tied to Irish identity both in Ireland and abroad. The symbolic resonance of the St. Patrick figure is complex and multifaceted, stretching from that of Christianity's arrival in Ireland to an identity that encompasses everything Irish. In some portrayals, the saint is symbolically synonymous with the Christian religion itself. There is also evidence of a combination of indigenous religious traditions with that of Christianity, which places St Patrick in the wider framework of cultural hybridity. Popular religious expression has this characteristic feature of merging elements of culture. Later in time, the saint becomes associated specifically with Catholic Ireland and synonymously with Irish national identity. Subsequently, St. Patrick is a patriotic symbol along with the colour green and the shamrock. St. Patrick's Day celebrations include many traditions that are known to be relatively recent historically, but have endured through time because of their association either with religious or national identity. They have persisted in such a way that they have become stalwart traditions, viewed as the strongest "Irish traditions".