Christianity arrived in Ireland sometime before the 5th century, presumably in interactions with Roman Britain. When the British gained full control of Ireland by means of a series of military campaigns in the period 1534–1691, the island was progressively colonised by English and Scottish Protestant settlers. Most of the Irish remained Roman Catholic.
Roman Catholicism is still the largest religious denomination, representing over 69% of the population of the island and about 69% of the population of the Republic of Ireland in the 2022 census.https://data.cso.ie/
Introduction of Christianity
The introduction of Christianity to Ireland dates to sometime before the 5th century, presumably in interactions with Roman Britain, though evidence also exists of early Christian influences stemming from the Coptic Orthodox Church in Egypt. Christian worship had reached pagan Ireland around AD 400. It is often claimed that Saint Patrick brought the faith to Ireland , but it was already present on the island before Patrick's mission.
Monasteries were built for monks who wanted permanent communion with God. The lengths to which some monks went for tranquillity are evident from the monastery of Skellig Michael. One notable Irish monk and missionary, Bishop Aidan (died 651), is credited with spreading Christianity among the Picts and Northumbrians.
Scholars have long regarded the term "Celtic Church" as inappropriate for describing Christianity among Celtic-speaking peoples, since this would imply a notion of unity, or a self-identifying entity, that did not exist.Dáibhí Ó Cróinín, Early Medieval Ireland 400–1200 (London, 1995); T. M. Charles-Edwards, Early Christian Ireland (Cambridge, 2000); W. Davies, "The Myth of the Celtic Church", in N. Edwards and A. Lane, The Early Church in Wales and the West (Oxbow Monograph 16, Oxford, 1992), pp. 12–21; Kathleen Hughes, "The Celtic Church: is this a valid concept?", in Cambridge Medieval Celtic Studies 1 (1981), pp. 1–20; Kathleen Hughes, The Church in Early English Society (London, 1966); W. Davies and P. Wormald, The Celtic Church (Audio Learning Tapes, 1980). As Patrick Wormald explained, "One of the common misconceptions is that there was a 'Roman Church' to which the 'Celtic' was nationally opposed."Patrick Wormald, "Bede and the 'Church of the English'", in The Times of Bede, ed. Stephen Baxter (Oxford: Blackwell Publishing, 2006), p. 207. Celtic-speaking areas were part of Latin Christendom as a whole, wherein a significant degree of liturgical and structural variation existed, along with a collective veneration of the Bishop of Rome that was no less intense in Britain and Ireland.Richard Sharpe, "Some problems concerning the organization of the Church in early medieval Ireland", Peritia 3 (1984), pp. 230–270; Patrick Wormald, "Bede and the 'Church of the English'", in The Times of Bede, ed. Stephen Baxter (Oxford: Blackwell Publishing, 2006), pp. 207–208, 220 n. 3 Some scholars have chosen to apply the term "Insular Christianity" to the Christian practice that arose around the Irish Sea. Others, most notably Eastern Orthodox apologists, believe that, up until the East-West Schism of 1054, Ireland effectively practised a provincial form of Eastern Orthodox Christianity.
Palladius and Patrick According to Prosper of Aquitaine, Palladius was from a noble family in Gaul. In 429, he was serving as a deacon in Rome. The Pope commissioned him to send Germanus, Bishop of Auxerre, to investigate rumours of Pelagianism in Britain. In 431, Pope Celestine I consecrated Palladius a bishop and sent him to minister to the "Scots believing in Christ". His mission mainly seems to have been to Irish Christians in the east midlands, Leinster, and perhaps east Munster. It is uncertain if he converted any Irish. What little is known of his mission is that it appears to have been successful, though it was later downplayed by partisans of Patrick.
Saint Patrick's dates are uncertain. All that can be said is that he was alive sometime in the fifth century, and was a missionary bishop rather than ministering to Christians. His areas appear to have been Ulster and north Connacht, but very little can be said with certainty about him. Later tradition from the seventh-century onwards is known to be unreliable.
Native ministries and Irish monasteries Monastic establishments arose in the sixth century, such as Clonard, founded by St. Finian, Clonfert by St. Brendan, Bangor by St. Comgall, Clonmacnoise by St. Kieran, Killeaney by St. Enda; and, in the seventh century, Lismore by St. Carthage and Glendalough by St. Kevin.
In 563, St. Columba, a native of Donegal, accompanied by a few companions, crossed the sea to Caledonia and founded a monastery on the desolate island of Iona. Further fresh arrivals came from Ireland and the monastery, with Columba as its abbot, was soon a flourishing institution, from which the Dalriadian Scots in the south and the Picts beyond the Grampians were evangelized. When Columba died in 597, Christianity had been preached and received in every district in Caledonia, and in every island along its west coast. In the next century Iona had so prospered that its abbot, St. Adamnan, wrote in excellent Latin the "Life of St. Columba". From Iona, the Irish Aidan and his Irish companions had gone south to evangelize Northumbria, Mercia, and Essex.
Missionaries abroad
Missionaries from Ireland to England and Continental Europe spread news of the flowering of learning, and scholars from other nations came to Irish monasteries. The excellence and isolation of these monasteries helped preserve Latin learning during the Early Middle Ages. The period of Insular art, mainly in the fields of illuminated manuscripts, metalworking, and sculpture flourished and produced such treasures as the Book of Kells, the Ardagh Chalice, and the many carved stone crosses that dot the island.
These monasteries served as sanctuary to many of the continent's great scholars and theologians. It was here that the lamp of Latin learning was preserved for the ages. During this age, the great illuminated manuscripts of Ireland were produced. Arguably the finest such work is The Book of Kells which may still be viewed at Trinity College, Dublin.
The first significant renewal of learning in the West came with the Carolingian Renaissance of the Early Middle Ages. Charlemagne, advised by Peter of Pisa and Alcuin of York, attracted the scholars of England and Ireland, and by decree in AD 787 established schools in every abbey in his empire. These schools, from which the name scholasticism is derived, became centres of medieval learning. During the early Scholastic period, knowledge of the Greek language had vanished in the West except in Ireland, where it was widely dispersed in the monastic schools.MacManus, p 215
Irish scholars had a considerable presence in the Frankish court, where they were renowned for their learning. Among them was Johannes Scotus Eriugena, one of the founders of scholasticism.Toman, p 10: "Abelard himself was… together with John Scotus Erigena (9th century), and Lanfranc and Anselm of Canterbury (both 11th century), one of the founders of scholasticism." Eriugena was the most significant Irish intellectual of the early monastic period, and an outstanding philosopher in terms of originality. He had considerable familiarity with the Greek language, and translated many works into Latin, affording access to the Cappadocian Fathers and the Greek theological tradition.
Arrival of the Vikings During the ninth and tenth centuries, waves of Norse warriors ransacked the countryside. The monasteries were favourite targets for their treasures of golden religious ornaments.
As the eighth century neared its close, religion and learning still flourished, but unexpected dangers approached and a new enemy came, before whose assaults monk and monastery and saint and scholar disappeared. These invaders were the Danes from the coasts of Scandinavia. Pagans and pirates, they were formidable foes both on land and sea.
In Ireland as elsewhere they attacked the monasteries and churches, desecrated the altars, carried away the gold and silver vessels; smoking ruins and murdered monks attested to the fury of their assaults. Under native and Christian chiefs churches were destroyed, church lands appropriated by laymen, monastic schools deserted, and lay abbots ruled at Armagh and elsewhere. Bishops were consecrated without sees and conferred orders for money; there was chaos in church government and corruption everywhere.
In a series of synods beginning with Ráth Breasail (1118) and including Kells (1152), at which the Pope's legate presided, many salutary enactments were passed, and for the first time diocesan episcopacy was established. Meanwhile, St. Malachy, Archbishop of Armagh, had done very remarkable work in his own diocese and elsewhere. His early death in 1148 was a heavy blow to the cause of church reform. Nor could so many evils be cured in a single life, or by the labours of a single man; and in spite of his efforts and the efforts of others the decrees of synods were often flouted, and the new diocesan boundaries ignored.
Anglo-Normans In December 1154, Henry Plantagenet, Duke of Normandy and Aquitaine, by the Treaty of Wallingford and a large fleet, became in addition Henry II, King of England. In that same month an Englishman, Nicholas Breakspeare, was elected Pope Adrian IV. Henry intended, as shown by his later Constitutions of Clarendon, to establish the supremacy of the civil law and courts above the ecclesiastical law and courts. In the first full year of Henry's reign (1155) he procured the papal bull Laudabiliter from the Hertfordshire-born Pope Adrian IV, authorizing Henry to proceed to conquer Ireland "to check the torrent of wickedness to reform evil manners, to sow the seeds of virtue". The quid pro quo was the condition that a penny should be yearly paid from each house to the See of Rome (the still extant Peter's Pence). Henry and the Pope also had other reasons (see Henry II, section "Lordship over Ireland"). The invasion was put aside while Henry attended to other matters. Henry continued to battle the supreme power of the Church, and Thomas Becket in England. In 1166, Henry took the opportunity to accede to the request by displaced Irish Rí of Leinster, Diarmait Mac Murchada, for assistance in regaining his Irish territory. A first contingent of the Norman Invasion of Ireland came to Ireland in 1169 under Maurice Fitzgerald, followed by a stronger force under Strongbow (Richard de Clare, 2nd Earl of Pembroke) in 1170, the year Thomas à Beckett was murdered. In 1171 Henry himself landed at Waterford and proceeded to Dublin, where he spent the winter, and received the submission of all but two Irish chiefs. This submission was outlined in the Treaty of Windsor 1175.
Reformation and beyond It was not until the end of the 17th century that the English Crown later British Crown gained full control of Ireland by means of a series of military campaigns in the period 1534–1691. During this period, the island was progressively colonised by English and Scottish Protestant settlers. Most of the Irish remained Roman Catholic.
Henry VIII Henry VIII decided to destroy the power of the Anglo-Norman kings and take control of Ireland. As he did so, he put English lords in charge of confiscated land and plundered the Catholic monasteries and churches, as he had done in England. In 1536 during the Reformation, Henry arranged to be declared head of the Church in Ireland through an act of the Irish Parliament. When the Church of England was reformed under Edward VI, so too was the Church of Ireland.