The Coming of the Vikings
Who were the Vikings? They were a group of people who originated in modern-day Denmark and Norway. In the 700s, pressure on land in Scandanavia had forced many nobles and warriors to seek land elsewhere. Some of these were younger sons, who stood to inherit nothing of their father's estate. Noblemen with little to lose began to gather together groups of warriors and go down the coast pillaging settlements. They sold their booty for money, much like the black markets of today, and this became the means of making their living. The invention of the longboat made it possible for these warriors to sail across the North Sea to attack Britain,France and Ireland as well. In these areas they became known as the "Norsemen" (literally, north-men) and laterally as the "Vikings". They called themselves "Ostmen". The Vikings who first attacked Ireland were Norwegian while those in Britain were usually Danish. Being pagans, the Vikings did not have any respect for Christian symbols and sites. The picture on the left shows the Oseberg Ship, a reconstructed Viking raiding boat(Photo by Universitetets Oldsaksamling, Oslo).
The first raids in the British Isles was in 793, when the great monastery at Lindisfarne was sacked. In Ireland, Rathlin island monastery was burned by the Vikings in 795. Other prominent monasteries that were attacked included Holmpatrick, Inishmurray, Inishbofin and Sceilg Mhicil. Sceilg Mhicil's abbot died of thirst as a Viking prisoner. St Colum Cille's great monastery at Iona was burned in 802. For the next 30-40 years, the Vikings engaged in hit-and-run raids where they landed a small number of ships at a settlement, spent a few days pillaging and burning it before heading back to Scandanavia to sell their booty. The Vikings were after two types of booty - riches and slaves - which they carried off to sell. They soon found that the monasteries were the richest sources of both goods and this is why monasteries suffered so much. However, the Vikings also attacked a lot of grád Fhéne (commoner's) dwellings.
The brutality that the Vikings displayed towards their prisoners, and their apparent disrespect for anything other than booty must have injected terror into those who experienced, and heard tales of, the Norsemen's exploits. However, the effects of these raids should not be exaggerated. In this phase, there was about one attack per year and the probability of being attacked in any given year was actually quite low. Life went on as normal in Ireland. Nor did the Irish sit back and let the Vikings pillage their coasts. While most Irish attacks on the Vikings met with defeat, a few succeeded. The Ulaid defeated a band of raiders in 811, a band was defeated in Connaght in 812 and one in Munster around the same time.
The Raids Intensify
However, the Vikings were soon to improve their methods of pillaging. Instead of landing 3 or 4 boats, raiding nearby settlements and going back to Scandanavia, they decided to scale-up. They brought between 50 and 100 boats of Viking warriors, landed, and set up a camp. From this base they then raided extensively into the surrounding countryside for a period of several months. They pillaged monasteries, churches, the fortresses of Irish Lords, and farms. In 836 the lands of the southern Uí Néill suffered such an episode. In 837, the same thing happened on the Boyne and Liffey rivers on the east coast and on the Shannon on the west. In 840 the Vikings spent a year on Lough Neagh pillaging, amongst others, the monastery of Armagh. Many of the scholars and monks of Louth monastery were captured and sold into slavery. In 841 they set up fortified camps at Annagassan (county Louth) and Dubhlinn (present day Dublin). Clonmacnoise, Birr and Clonfert were pillaged and the primate of Armagh was captured and carried off in 845.
This was the most intense period of Viking activity, and the Irish Kings seemed to be able to do little to prevent the wholesale destruction of large tracts of their Provinces. The southern Uí Néill were routed by the Vikings when they attempted to drive them out. By the end, many of the monks themselves had taken to fighting the Vikings. However, just as it looked as if Ireland was about to be conquered by the Vikings, and just as the Irish began to develop tactics with which to more effectively attack them, the raids died away. The last major Viking raid of this phase was in 851 by which time they appeared to have turned their attention to Britain. The map below shows the attacks in this period.
Meanwhile, many of the Viking settlements developed and grew into towns. Their town of Dubhlinn had a thriving Norse community by the second half of the 800s, and had become the principal supplier of slaves in the British Isles. In time it became a great merchant town, until it was defeated by an Irish attack in 902. After that, the Vikings moved their power base to the Isle of Man and to the growing territory that the Vikings were carving out of Anglo-Saxon England. Other Viking towns had also been defeated, for example Cork in 848, Vadrefjord [Waterford] in 864 and Youghal in 866.
The Second Period of Raids
A second phase of raiding began in 914, with the arrival of a large fleet of Viking ships in Waterford harbour. They promptly re-captured their settlement of Vadrefjord [Waterford] from which the Irish had expelled the first Vikings half a century earlier. Reinforced by a second fleet which arrived the following year, the Vikings launched a series of offensives deep into the province of Munster, and later Leinster, where they met little Irish resistance as they pillaged both ecclesiastical and grád Fhéne (commoner) settlements. They plundered the monasteries of Cork, Lismore and Aghaboe, among others.In 917, the Vikings re-captured the settlement of Dubhlinn [Dublin] which the Irish had captured in 902. The king of the Uí Néill, Niall Glúndub, who was the most powerful king in Ireland, decided that the Vikings had to be stopped. He brought together a combined force from the Uí Néill and enlisted the help of the forces of Leinster. They marched against the Vikings in Munster in 917. However, the Vikings routed the Leinstermen, while the forces of the Uí Néill retreated from Munster with no decisive success. Two years later, in 919, Niall Glúndub tried again and attacked Dubhlinn. However, his forces were again routed by the Vikings and Niall Glúndub himself was killed and "the cream of the Uí Néill fell with him" . It was not true to say that it was "the Irish against the Vikings". In fact, some Irish kings and lords formed alliances with Vikings to attack other Irish lords.
The Vikings continued to raid inland from their towns of Dubhlinn, Cork and Vadrefjord. In 921, they founded a new town on the south-east tip of Ireland called Weisfjord (Wexford) and a year later founded the town of Limerick near a ford at the mouth of the river Shannon on the west coast. The Vikings in Ireland, however, spent a lot of effort consolidating the Nordic Kingdom that their Viking collegeaues had been carving out of Anglo-Saxon England (by defeating and assimilating Northumbria, East Anglia and parts of Mercia - see a map of England before the Vikings came). This kingdom would become known as the Danelaw. Back in Ireland, as the influence of the Vikings declined, they concentrated more on developing Dubhlinn as a trading city and by 934 exercised control over the other Viking towns in Ireland. In its day, Dubhlinn was one of the most important cities in the Nordic world, as a trading and slaving centre. In 952, Dubhlinn split from the Danelaw and from then on Dubhlinn had its own dynasty of Viking Kings.
See below for a map of Ireland around 950.
The Vikings eventually settled down in the lands they had conquered. By 950, the Vikings had stopped raiding in Ireland and developed instead as traders and settled in the lands around their towns. The Vikings in England  largely became farmers and fishermen. In France, the Vikings formed the Kingdom of Normandy on the north coast - which would play a major role in history a century later when William of Normandy would defeat England in 1066. The Vikings left many placenames in Ireland including: Dublin, Cork, Limerick, Waterford, Wexford, Strangford, Leixlip, Carlingford, Youghal, Howth, Dalkey and Fingall [an area of modern-day Dublin]. A few of their words were also adopted into the Irish language.
The First High Kings of Ireland
Despite these Viking attacks, life continued as normal in Ireland's provinces. As discussed in the previous section, the power of the Uí Néill rose during the 700s and this continued into the 800s. After conquering the province of Airgialla (central Ulster) between 750 and 850, the Northern Uí Néill turned their attention to the eastern province of Ulaid. The Ulaid, recognising the supremacy of the Uí Néill, did not attempt to resist and they were under Uí Néill control by the mid 800s. The Northern Uí Néill themselves were ruled by the Cenél nEógain family, and they were bitterly resented by the Cenél Conaill of western Ulster and the Ulaid in the east. The southern Uí Néill, on the other hand, had gained control over northern Laigin. Split by the expansion of Connacht into Bréifne in the 700s, the two halves of the Uí Néill were united again, in the east, by the end of the 800s.Later writings referred to the kings of the Uí Néill as the first High Kings of Ireland, but it seems unlikely that this in reality referred to anything more than an aspiration. After 940, a bitter power struggle broke out between the royal families of the Uí Néill. Foster  sums this up as "a united Uí Néill kingdom was in the making, and the struggle was to determine who was to be the ruler of it". The King of the Northern Uí Néill, Domnall ua Néill, who was also the overall Uí Néill king, attempted to rule the Southern Uí Néill directly and even garrisoned forces in their territory. The next king was Mael Sechnaill II of the Southern branch. The Uí Néill had gone from being an obscure people in western Ireland to the rulers or controllers of most of northern and eastern Ireland. Although they never really exercised control over Connacht or Munster, their later proponents preferred to style them as the first High Kings.
The map below shows how Ireland looked like around 950AD. (This makes an interesting comparison with the map of Ireland three centuries earlier, before the Uí Néill expansion.)