In his sixteenth year, Patrick was carried off into captivity by Irish marauders and was sold as a slave to a chieftain named Milchu in Dalriada, a territory of the present county of Antrim in Ireland, where for six years he tended his master's flocks in the valley of the Braid and on the slopes of Slemish, near the modern town of Ballymena. He relates in his "Confessio" that during his captivity while tending the flocks he prayed many times during the day: "the love of God", he added,

"…and His fear increased in me more and more, and the faith grew in me, and the spirit was roused, so that, in a single day, I have said as many as a hundred prayers, and in the night nearly the same, so that whilst in the woods and on the mountain, even before the dawn, I was roused to prayer and felt no hurt from it, whether there was snow or ice or rain; nor was there any slothfulness in me, such as I see now, because the spirit was then fervent within me."

In the ways of a benign Providence the six years of Patrick's captivity became a remote preparation for his future apostolate. He acquired a perfect knowledge of the Celtic tongue in which he would one day announce the glad tidings of Redemption, and, as his master Milchu was a druidical high priest, he became familiar with all the details of Druidism from whose bondage he was destined to liberate the Irish race.

Admonished by an angel he after six years fled from his cruel master and bent his steps towards the west. He relates in his "Confessio" that he had to travel about 200 miles; and his journey was probably towards Killala Bay and onwards thence to Westport. He found a ship ready to set sail and after some rebuffs was allowed on board. In a few days he was among his friends once more in Britain, but now his heart was set on devoting himself to the service of God in the sacred ministry. We meet with him at St. Martin's monastery at Tours, and again at the island sanctuary of Lérins which was just then acquiring widespread renown for learning and piety; and wherever lessons of heroic perfection in the exercise of Christian life could be acquired, thither the fervent Patrick was sure to bend his steps. No sooner had St. Germain entered on his great mission at Auxerre than Patrick put himself under his guidance, and it was at that great bishop's hands that Ireland's future apostle was a few years later ordained to the priesthood. It is the tradition in the territory of the Morini that Patrick under St. Germain's guidance for some years was engaged in missionary work among them. When Germain commissioned by the Holy See proceeded to Britain to combat the erroneous teachings of Pelagius, he chose Patrick to be one of his missionary companions and thus it was his privilege to be associated with the representative of Rome in the triumphs that ensued over heresy and Paganism, and in the many remarkable events of the expedition, such as the miraculous calming of the tempest at sea, the visit to the relics at St. Alban's shrine, and the Alleluia victory. Amid all these scenes, however, Patrick's thoughts turned towards Ireland, and from time to time he was favored with visions of the children from Focluth, by the Western sea, who cried to him: "O holy youth, come back to Erin, and walk once more amongst us."

Pope St. Celestine I entrusted St. Patrick with the mission of gathering the Irish race into the one fold of Christ. Palladius had already received that commission, but terrified by the fierce opposition of a Wicklow chieftain had abandoned the sacred enterprise. It was St. Germain, Bishop of Auxerre, who commended Patrick to the pope. Patrick on his return journey from Rome received episcopal consecration at the hands of its great bishop, St. Maximus, and then hastened on to Auxerre to make under the guidance of St. Germain due preparations for the Irish mission.

It was probably in the summer months of the year 433, that Patrick and his companions landed at the mouth of the Vantry River close by Wicklow Head. The Druids were at once in arms against him. But Patrick was not disheartened. The intrepid missionary resolved to search out a more friendly territory in which to enter on his mission. First of all, however, he would proceed towards Dalriada, where he had been a slave, to pay the price of ransom to his former master, and in exchange for the servitude and cruelty endured at his hands to impart to him the blessings and freedom of God's children. He rested for some days at the islands off the Skerries coast, one of which still retains the name of Inis-Patrick, and he probably visited the adjoining mainland, which in olden times was known as Holm Patrick. Tradition fondly points out the impression of St. Patrick's foot upon the hard rock -- off the main shore, at the entrance to Skerries harbor. Continuing his course northwards he halted at the mouth of the River Boyne. A number of the natives there gathered around him and heard with joy in their own sweet tongue the glad tidings of Redemption. There too he performed his first miracle on Irish soil to confirm the honour due to the Blessed Virgin, and the Divine birth of our Saviour. Leaving one of his companions to continue the work of instruction so auspiciously begun, he hastened forward to Strangford Loughand there quitting his boat continued his journey over land towards Slemish. He had not proceeded far when a chieftain, named Dichu, appeared on the scene to prevent his further advance. He drew his sword to smite the saint, but his arm became rigid as a statue and continued so until he declared himself obedient to Patrick. Overcome by the saint's meekness and miracles, Dichu asked for instruction and made a gift of a large sabhall (barn), in which the sacred mysteries were offered up. This was the first sanctuary dedicated by St. Patrick in Erin. It became in later years a chosen retreat of the saint. A monastery and church were erected there, and the hallowed site retains the name Sabhall (pronounced Saul) to the present day. Continuing his journey towards Slemish, the saint was struck with horror on seeing at a distance the fort of his old master Milchu enveloped in flames. The fame of Patrick's marvelous power of miracles preceded him. Milchu, in a fit of frenzy, gathered his treasures into his mansion and setting it on fire, cast himself into the flames. An ancient record adds: "His pride could not endure the thought of being vanquished by his former slave".

Returning to Saul, St. Patrick learned from Dichu that the chieftains of Erin had been summoned to celebrate a special feast at Tara by Leoghaire, who was the Ard-Righ, that is, the Supreme Monarch of Ireland. This was an opportunity which Patrick would not forego; he would present himself before the assembly, to strike a decisive blow against the Druidism that held the nation captive, and to secure freedom for the glad tidings of Redemption of which he was the herald. As he journeyed on he rested for some days at the house of a chieftain named Secsnen, who with his household joyfully embraced the Faith. The youthful Benen, or Benignus, son of the chief, was in a special way captivated by the Gospel doctrines and the meekness of Patrick. Whilst the saint slumbered he would gather sweet-scented flowers and scatter them over his bosom, and when Patrick was setting out, continuing his journey towards Tara, Benen clung to his feet declaring that nothing would sever him from him. "Allow him to have his way", said St. Patrick to the chieftain, "he shall be heir to my sacred mission." Thenceforth Benen was the inseparable companion of the saint, and the prophecy was fulfilled, for Benen is named among the "comhards" or successors of St. Patrick in Armagh.

It was on 26 March, Easter Sunday, in 433, that the eventful assembly was to meet at Tara, and the decree went forth that from the preceding day the fires throughout the kingdom should be extinguished until the signal blaze was kindled at the royal mansion. The chiefs and Brehons came in full numbers and the druids too would muster all their strength to bid defiance to the herald of good tidings and to secure the hold of their superstition on the Celtic race, for their demoniac oracles had announced that the messenger of Christ had come to Erin. St. Patrick arrived at the hill of Slane, at the opposite extremity of the valley from Tara, on Easter Eve, in that year the feast of the Annunciation, and on the summit of the hill kindled the Paschal fire. The druids at once raised their voice. "O King", (they said) "live for ever; this fire, which has been lighted in defiance of the royal edict, will blaze for ever in this land unless it be this very night extinguished." By order of the king and the agency of the druids, repeated attempts were made to extinguish the blessed fire and to punish with death the intruder who had disobeyed the royal command. But the fire was not extinguished and Patrick shielded by the Divine power came unscathed from their snares and assaults. On Easter Day the missionary band having at their head the youth Benignus bearing aloft a copy of the Gospels, and followed by St. Patrick who with miter and crosier was arrayed in full episcopal attire, proceeded in processional order to Tara.

Thus was the final blow given to paganism in the presence of all the assembled chieftains.

The beautiful prayer of St. Patrick, popularly known as "St. Patrick's Breast-Plate", is supposed to have been composed by him in preparation for this victory over Paganism. The following is a literal translation from the old Irish text:

I bind to myself today
The strong virtue of the Invocation of the Trinity:
I believe the Trinity in the Unity
The Creator of the Universe.

I bind to myself today
The virtue of the Incarnation of Christ with His Baptism,
The virtue of His crucifixion with His burial,
The virtue of His Resurrection with His Ascension,
The virtue of His coming on the Judgment Day.

I bind to myself today
The virtue of the love of seraphim,
In the obedience of angels,
In the hope of resurrection unto reward,
In prayers of Patriarchs,
In predictions of Prophets,
In preaching of Apostles,
In faith of Confessors,
In purity of holy Virgins,
In deeds of righteous men.

I bind to myself today
The power of Heaven,
The light of the sun,
The brightness of the moon,
The splendor of fire,
The flashing of lightning,
The swiftness of wind,
The depth of sea,
The stability of earth,
The compactness of rocks.

I bind to myself today
God's Power to guide me,
God's Might to uphold me,
God's Wisdom to teach me,
God's Eye to watch over me,
God's Ear to hear me,
God's Word to give me speech,
God's Hand to guide me,
God's Way to lie before me,
God's Shield to shelter me,
God's Host to secure me,
Against the snares of demons,
Against the seductions of vices,
Against the lusts of nature,
Against everyone who meditates injury to me,
Whether far or near,
Whether few or with many.

I invoke today all these virtues
Against every hostile merciless power
Which may assail my body and my soul,
Against the incantations of false prophets,
Against the black laws of heathenism,
Against the false laws of heresy,
Against the deceits of idolatry,
Against the spells of women, and smiths, and druids,
Against every knowledge that binds the soul of man.

Christ, protect me today
Against every poison, against burning,
Against drowning, against death-wound,
That I may receive abundant reward.

Christ with me, Christ before me,
Christ behind me, Christ within me,
Christ beneath me, Christ above me,
Christ at my right, Christ at my left,
Christ in the fort,
Christ in the chariot seat,
Christ in the poop [deck],
Christ in the heart of everyone who thinks of me,
Christ in the mouth of everyone who speaks to me,
Christ in every eye that sees me,
Christ in every ear that hears me.

I bind to myself today
The strong virtue of an invocation of the Trinity,
I believe the Trinity in the Unity
The Creator of the Universe.

The manifold virtues by which the early saints were distinguished shone forth in all their perfection in the life of St. Patrick. When not engaged in the work of the sacred ministry, his whole time was spent in prayer. Many times in the day he armed himself with the sign of the Cross. He never relaxed his penitential exercises. Clothed in a rough hair-shirt, he made the hard rock his bed. His disinterestedness is specially commemorated. Countless converts of high rank would cast their precious ornaments at his feet, but all were restored to them. He had not come to Erin in search of material wealth, but to enrich her with the priceless treasures of the Catholic Faith.

St. Patrick, having now completed his triumph over Paganism, and gathered Ireland into the fold of Christ, prepared for the summons to his reward with God. At Saul (Sabhall), St. Patrick received the summons to his reward on 17 March, 493 St. Tassach administered the last sacraments to him. His remains were wrapped in the shroud woven by St. Brigid's own hands. The bishops and clergy and faithful people from all parts crowded around his remains to pay due honor to the Father of their Faith. Some of the ancient Lives record that for several days the light of heaven shone around his bier. His remains were interred at the chieftain's Dun or Fort two miles from Saul, where in after times arose the cathedral of Down.