The Scandinavians raided Lambey Island in Dublin Bay in 795 and persisted in their assaults for another half century. A primary objective were the monasteries, where they sought precious metals and jewels. Attaching no value to books, they destroyed them. When they found no treasure, they took food and slaves. They also took venerated items, such as relics, and individuals for ransom. They had attacked Colm Cille's settlement, Iona, in northern Scotland, Bangor, and Skelligs. They even traveled inland, attacking and sacking places like Armagh. Norse fleets appeared on the Boyne, the Liffey, and the Shannon. Dread of the Vikings prompted lamentations and prayers such as "From the fury of the Northmen deliver us O Lord," which evocation would be repeated elsewhere in Western Europe for more than a century. The destruction of so many monasteries in Ireland by the Vikings forced many Irish monks to transfer to the Continent, where, like John Scotus, they assumed positions of importance as scholars and missionaries. This flight to continental Europe might have diminished the glories of monasticism in Ireland itself. Significantly, the Irish monks on the Continent began to accept the Benedictine rule and the ascendancy of the episcopacy.
By the middle of the ninth century, the Vikings began to winter in Ireland and establish permanent settlements. Those who arrived first concentrated along the east coast in primarily military outposts from which to launch raids elsewhere. In some cases the Vikings brought their own women to these settlements, but eventually there was a certain amount of intermarriage with Irish women. Likewise, there was even a certain degree of cooperation or collaboration with Irish chieftains who did not feel necessarily loyal to other Irish chieftains, especially if they believed alliance with the Vikings would be to their individual benefit in terms of expansion of territory or riches. The final stage of integration or permanency would be the Vikings' acceptance of Christianity. Possibly the most significant or lasting contribution of the Vikings to Irish history was the gradual transformation of their military settlements into the first Irish cities. Places like Dublin, Limerick, Waterford, Wexford, and Cork owe their origins to the Vikings. Traders, who brought commercial life to a primarily agrarian island, came after the Viking raiders. Significantly, the use of coinage followed the Viking settlement. On the other hand, the cultural contributions were minimal. Few words of Viking origin remain, and there were not significant Viking influences in artistic work.
Paradoxically, the greatest contribution of the Vikings might well have been the reaction to them by some Irish kings, who utilized the struggle against them to enhance the concept of high king and even the idea of an Irish nation. This occurred at about the end of the first millennium when the Vikings had increasingly accepted the Christian religion of the Irish. The central figure in this development was the heroic figure Brian Boru, whom legend and history have combined to magnify as someone comparable to Alfred the Great of Anglo-Saxon England as the savior or liberator of the nation from the Vikings. Of course, the story is somewhat more complex and less ideal. Brian Boru was of the Dal Cais, a Munster royal family that challenged the power of a Viking ruler in Limerick as well as the Eoganacht kings of Cashel, who traditionally had been matched only by the Ui Neill of the North in rank. Brian's ambitions might have been prompted by the treacherous murder of his brother, whom he had succeeded. At any rate, during the last quarter of the 10th century he had gained such ascendancy over various other kingdoms in Ireland that even the Ui Neill formally acknowledged him as the over-king of the southern half of the island, and ultimately as the high king of all Ireland, noting in the precious Book of Armagh, on the occasion of Brian's visit there in 1005, that he was the emperor of the Irish.
In 1014 Brian went on the offensive against the king of Leinster and a Viking king, both of whom refused to acknowledge his supremacy. In the fierce Battle of Clontarf on Good Friday 1014, in which there were many casualties and in which most of the leaders, including Brian himself, were killed, the Irish won. That victory might well have inhibited a possible renewed Viking assault on the island, as the year before the king of Denmark had conquered England (his son Canute would later be king of England), and a great Viking fleet had raided Munster. Also, although the high kingship remained more theoretical than real, it was on the way to becoming a reality. In less than two centuries that reality would come about, but it would be exercised by King Henry II of England. Before then the Irish church would experience the reforming currents then at work in the continental church.
Earlier, in the seventh and eight centuries, Ireland was in the forefront of intellectual and cultural development in a retrogressive Europe. However, by the 11th and 12 th centuries, the island had become a backwater to which the creative and reforming energies of the Continent were barely making their way. Central to what was happening in Europe was the growing ascendancy within the church of the reform party, which sought, in the spirit of the monk Hildebrand, later Pope Gregory VII, to free the church from secular dominance, that is, from private or familial control of church offices and property. The specific offenses fought against were simony, the sale of offices, and lay investiture, the awarding of church offices, such as bishoprics and abbacies, by political figures rather than by church authorities. A reinvigorated papacy, no longer selected by the nobility of Rome, but instead by a more universal electoral body, the College of Cardinals, was able to assume the reforming task. However, to rid itself of control by secular leaders, the church had to rely on other secular leaders, who, over time, would prove as troublesome to the church. At the time, the leaders most likely to respond to the desire to free the church from control by local leaders were kings or national monarchs who were increasingly asserting their own authority against more localized feudal lords. Unfortunately, despite Brian Boru's achievements, the high kings of Ireland, or any of the rival claimants to the title, were nowhere near neither the kings of France or England nor the Holy Roman Emperor in being able to assert any centralized authority.
The church in Ireland also languished in a pattern of weakened bishops and abbots greatly dependent on local kings and holding titles by reason of family. In addition, the century and a half of Viking plunder had diminished the spiritual and cultural grandeur of Irish monasticism. Significantly, the earlier moves in the direction of the continental reform movement within Ireland occurred in dioceses, such as Dublin and Waterford, that were settlements of by now Christian Vikings. There the reforming archbishop of Canterbury, Lanfranc, and his later successor, Anselm, had consecrated the bishops. The ascendancy of the reforming party in England had followed the conquest of England by William the Conqueror of Normandy in 1066. That conquest began the completion of feudalization in England, but in a manner that augmented the centralizing authority of the national monarch. In the 11th century the reforming party advanced in Ireland. The reform cause benefited from the sympathy of Turloch O'Connor, the king of Connacht, who now wielded as much ascendant power as high king as had Brian Boru a century and a half before. He presided over the Synod of Kells in 1152, which advanced the organizational and disciplinary reform of the church in Ireland in accord with the spirit desired by the papacy.
The churchman most connected with this was the archbishop of Armagh, Malachy (St. Malachy), who had gone to Rome several years before seeking formal papal acceptance of his title to the office. On his way to Rome he met St. Bernard of Clairvaux, the leader of the reforming monastic order of Cistercians, whom he informed of the church's situation in Ireland. The pope, Innocent II, was willing to acknowledge Malachy's position, but only after a national synod in which the reforms would be implemented. That synod, which occurred after Malachy's death, finally took place with Turloch O'Connor's approval. It organized the Irish church into 36 dioceses and four archbishoprics, Armagh, Cashel, Tuam, and Dublin, and the administrative arrangement was acknowledged by Rome. Even before the synod, Malachy had brought with him a band of Cistercians who established their monastery in Mellifont, the first Gothic structure in Ireland. From then on European-style monastic organization began to take hold in Ireland.