The one living thing in the social and political chaos was the sept, or tribe, or clan, whose institutions remained those of the earliest stage of human civilization. Its chieftainship was hereditary, but, instead of passing from father to son, it was held by whoever was the eldest member of the ruling family at the time. The land belonging to the tribe was shared among its members, but re-divided among them at certain intervals of years. The practice of "fosterage," or adoption, bound the adopted child more closely to its foster-parents than to its family by blood. Every element of improvement or progress which had been introduced into the island disappeared in the long and desperate struggle with the Danes. The coast-towns, such as Dublin or Waterford, which the invaders founded, remained Danish in blood and manners, and at feud with the Celtic tribes around them, though sometimes forced by the fortunes of war to pay tribute, and to accept, in name at least, the overlordship of the Irish Kings. It was through these towns however that the intercourse with England, which had ceased since the eighth century, was to some extent renewed in the eleventh.

Cut off from the Church of the island by national antipathy, the Danish coast-cities applied to the See of Canterbury for the ordination of their bishops, and acknowledged a right of spiritual supervision in Lanfranc and Anselm. The relations thus formed were drawn closer by the slave-trade, which the Conqueror and Bishop Wulfstan succeeded for a time in suppressing at Bristol, but which appears to have quickly revived. In the twelfth century Ireland was full of Englishmen, who had been kidnapped and sold into slavery, in spite of royal prohibitions and the spiritual menaces of the English Church. The state of the country afforded a legitimate pretext for war, had a pretext been needed by the ambition of Henry the Second; and within a few months of that King's coronation John of Salisbury was despatched to obtain the Papal sanction for an invasion of the island. The enterprise, as it was laid before Pope Hadrian the Fourth, took the colour of a crusade. The isolation of Ireland from the general body of Christendom, the absence of learning and civilization, the scandalous vices of its people, were alleged as the grounds of Henry's action.

It was the general belief of the time that all islands fell under the jurisdiction of the Papal See, and it was as a possession of the Roman Church that Henry sought Hadrian's permission to enter Ireland. His aim was "to enlarge the bounds of the Church, to restrain the progress of vices, to correct the manners of its people and to plant virtue among them, and to increase the Christian religion." He engaged to "subject the people to laws, to extirpate vicious customs, to respect the rights of the native Churches, and to enforce the payment of Peter's pence " as a recognition of the overlordship of the Roman See. Hadrian by his bull approved the enterprise as one prompted by "the ardour of faith and love of religion," and declared his will that the people of Ireland should receive Henry with all honour, and revere him as their lord. The Papal bull was produced in a great council of the English baronage, but the opposition of the Empress Matilda and the difficulties of the enterprise forced on Henry a temporary abandonment of his designs, and his energies were diverted for the moment to plans of continental aggrandizement. Twelve years had passed when an Irish chieftain.

Dermod, King of Leinster, presented himself at Henry's Court, and did homage to him for the dominions from which he had been driven in one of the endless civil wars which distracted the island. Dermod returned to Ireland with promises of aid from the English knighthood; and was soon followed by Robert FitzStephen, a son of the Constable of Cardigan, with a small band of a hundred and forty knights, sixty men-at-arms, and three or four hundred Welsh archers. Small as was the number of the adventurers, their horses and arms proved irresistible to the Irish kernes; a sally of the men of Wexford was avenged by the storm of their town; the Ossory clans were defeated with a terrible slaughter, and Dermod, seizing a head from the heap of trophies which his men piled at his feet, tore off in savage triumph its nose and lips with his teeth. The arrival of fresh forces under Maurice Fitzgerald heralded the coming of Richard of Clare, Earl of Pembroke and Striguil, a ruined baron later known by the nickname of Strongbow, who in defiance of Henry's prohibition landed near Waterford with a force of fifteen hundred men, as Dermod's mercenary.

The city was at once stormed, and the united forces of the Earl and King marched to the siege of Dublin. In spite of a relief attempted by the King of Connaught, who was recognized as overking of the is-land by the rest of the tribes, Dublin was taken by surprise; and the marriage of Richard with Eva, Dermod's daughter, left him on the death of his father-in-law, which followed quickly on these successes, master of his kingdom of Leinster. The new lord had soon, however, to hurry back to England, and appease the jealousy of Henry by the surrender of Dublin to the Crown, by doing homage for Leinster as an English lordship, and by accompanying the King in his voyage to the new dominion which the adventurers had won. Had Henry been allowed by fortune to carry out his purpose, the conquest of Ireland would now have been accomplished. The King of Connaught indeed and the chiefs of northern Ulster refused him homage, but the rest of the Irish tribes owned his suzerainty; the bishops in synod at Cashel recognized him as their lord; and he was preparing to penetrate to the north and west, and to secure his conquest by a systematic erection of castles throughout the country, when the troubles which followed on the murder of Archbishop Thomas recalled him hurriedly to Normandy. The lost opportunity never returned.