Excert from
http://www.irelandseye.com/aarticles/history/events/dates/ss.shtm
Ireland, situated at the western edge of Europe, was certainly inhabited as early as 6,000 BC. Archaeology provides evidence of successive invasions, hunters and fishermen followed by farmers using stone tools to clear land, then by tribes skilled in metalwork. The last pre-Christian invaders were the iron-using Celts or Gaels, who reached Ireland around 600 BC.
More than 1,000 years later, it was a well-established pagan Celtic society which accepted Christianity.
Fifth-century Ireland was divided into a number of small kingdoms. The more influential kings received tribute from the weaker ones, but the idea of a high king of Ireland is a later invention.
The Celts were cattle farmers, wealthy enough to devote some resources to intricate gold and silver ornaments, sophisticated enough to have lawyers (brehons) and poets (filidh) as well as the druids who practised magic and offered sacrifices to the pagan gods. They had their own language, from which modern Irish has evolved. Kings were elected, but from a narrowly defined group possessing royal blood. Wars were common, and the balance of power between kingdoms shifted constantly.
Saint Patrick divided Ireland into dioceses, but before long abbots became more influential than bishops. The monasteries were important seats of learning at a time when the European mainland was entering a dark age following the collapse of the Roman Empire. In time Irish monks set out to spread the Christian message in foreign countries. Among them were Saint Columba, who founded a famous monastery on the Scottish island of Iona, and Saint Columbanus, who founded monasteries in France, Germany and Italy.
Ireland had escaped invasion by the Roman legions, but it could not escape the Viking longboats which menaced its coasts and rivers in the ninth century. But, while the Norsemen sacked monasteries, they also settled as traders and founded settlements which grew into such cities as Dublin, Cork and Limerick.
The Celtic kingdoms eventually fought back, and in 1014 Brian Boru, who had claimed the high kingship, won a decisive victory over the Norsemen at Clontarf, near Dublin.
By now the Church was in disarray, the monasteries corrupt and the dioceses ineffective. However, the establishment of Cistercian monasteries from 1142 on, and the reorganisation of the Irish Church at the Synod of Kells in 1152 set reforms in motion. The petty kingdoms remained a source of disunion. Brian Boru had been killed at Clontarf, and successive aspirants to the high kingship were unable to enforce their authority.
The land of saints and scholars lacked military and political cohesion, and was in no condition to repel the next invaders who sailed for Ireland.