In the year 1169, Fitz-Stephen, with his companions, accompanied by 140 knights and 300 men-at-arms, crossed over to Ireland, and landed at Bannock Bay. MacMurrogh, who had previously returned to the country, and had remained in concealment, advanced to meet his friends. The combined forces having attacked and reduced Wexford, advanced against the Prince of Ossory, whom they defeated with great slaughter. The Normans slew their adversaries, who possessed no defensive armour, and cut off their heads with their battle-axes. It is related that three hundred bleeding heads were brought and laid before MacMurrogh, and that he turned them over to see which of his enemies had been slain. On coming to the head of one against whom he had a mortal hatred, he took it up by the hair, and, "horribly and cruelly, tore away the nose and lips with his teeth." This savage chieftain, however, had a regard for his plighted word, and he fulfilled his promise of placing Fitz-Stephen in possession of Wexford, while districts on the coast between Waterford and Wexford were given to others of his allies. These gifts of territory to foreigners called forth the utmost indignation among the Irish confederate chiefs, who, at a council held at the royal seat of Tara, in Meath, declared the King of Leinster to be a national enemy, and prepared to make common cause against him.

Strongbow, Earl of Pembroke, did not set sail for Ireland until 1170. He landed near to Waterford, with a force of two hundred knights and two thousand men, and was immediately joined by the Normans who had preceded him. The combined forces, having been arranged in battle array, and with banners flying, advanced to attack the city. The citizens made a gallant resistance, and were probably excited to desperation by the ruthless character of MacMurrogh, and the fate which they expected would await those who might fall into his hands. The Earl of Pembroke, who was well skilled in the art of war, had command of the forces, and led the assault. A little house of timber, standing half upon posts, was observed without the walls, and the assailants having hewn down the posts, the house fell, together with a piece of the wall. The troops poured through the breach thus made, and captured the city, killing the inhabitants without mercy.

Leaving a strong garrison, the Normans marched to Dublin, which town, as well as that of Waterford, had been founded by the Danes. Supported by reinforcements raised by MacMurrogh, the invaders took the city of Dublin with little resistance, and, elated by a course of uninterrupted successes, made incursions upon the surrounding country. King Henry, however, received the news of these events, and his jealousy being excited at such an important conquest being attained by his vassals, he issued a proclamation forbidding any vessel to leave his dominions for Ireland, and ordered all his subjects then in that country to return to England by the next Easter, on pain of the forfeiture of all their estates, and of perpetual banishment from the realm. A consultation was held among the Normans, and Raymond Fitz-William, surnamed Le Gros, nephew of Fitz-Gerald and Fitz-Stephen, was dispatched on a mission to Henry, to prevail upon him to recall the proclamation, and to remind him of the letters he had given to MacMurrogh, authorising Englishmen to take up arms in his cause. Henry received the message without returning any answer, or, according to some of the chroniclers, he replied by confiscating the estates of Strongbow in Wales.

While the earl thus found himself cut off from all reinforcements of men and arms, the Normans in Leinster were suddenly attacked by the men of Danish race who were settled on the north-east coast of Ireland, and who now allied themselves with the natives against the new invaders. They attacked Dublin, but without success. The Normans, however, dreading the formidable league against them, made a second application to Henry through Hervey Fitz-Maurice. Strongbow himself was then ordered to proceed to the court, and after some delay he obtained an audience. The earl agreed to surrender to the king the town of Dublin, with the larger of the other towns on the coast; in return, Strongbow was permitted to retain his other acquisitions in Ireland, and was restored to the possession of his estates in Wales.

MacMurrogh having died previously to this interview, Strongbow had assumed the title of King of Leinster, in right of his wife Eva; and he now found himself reduced from the condition of a sovereign prince to that of steward of the English crown. In the year 1171, Henry set sail from Milford to take possession of his new territories. The royal force consisted of 400 vessels, containing about 5,000 men, among whom were 500 knights. Henry landed at the Crook, near Waterford, October 18th, and was received by the Norman chiefs, who tendered him their homage. The army commenced its march, by way of Cashel, to Dublin, meeting with no resistance. The inhabitants, overawed by the numbers and the martial equipment of their enemies, fled in dismay before the advancing troops, and the native kings of the south had no other alternative than to surrender at the summons of the conqueror, and offer their allegiance to him.

Having established his court at Dublin, Henry styled himself King of all Hibernia, and summoned the whole of the Irish chiefs to his presence. Many obeyed; but the Kings of Connaught and Ulster, entrenched in their native mountains, refused to acknowledge his authority, and the sovereignty of Henry was limited by a line drawn across the island, from the mouth of the Shannon to that of the Boyne. All the pomp which distinguished the Plantagenet court was displayed in Dublin, and the Irish people—lively, impressible, and fond of novelty—derived pleasure from contemplating the splendid appearance of the Norman arms, horses, and accoutrements of war. The majority of the clergy also gave their support to the invader, and welcomed him as one bearing the authority of the Church. Henry promulgated the bull of Pope Adrian; and various reforms and observances of canonical discipline were introduced into the Irish Church.

Henry's former haughtiness towards the clergy, and his resistance to the encroachments of the papal see upon the rights of the crown, had now disappeared. Not only did he require the support of the bishops to secure his new conquest, but the popular feeling excited throughout his dominions by the death of Becket rendered it necessary for him to conciliate where he had formerly threatened. This course of action met with temporary success, and Pope Alexander III. issued a bull confirming that of his predecessor, Adrian, and ratifying the king's title to the possession of Ireland.

After he had remained in the country for a few months longer, Henry received news which compelled his immediate return to England. Having appointed officers to the chief places of power in the island, he sailed from Wexford on the 17th of April, 1172, and landed at Portfinnan, in Wales.

At this time the king had four legitimate sons living—Henry, Richard, Geoffrey, and John, of whom Henry, the eldest, was eighteen years of age. Equitable provision had been made for each of them, it being intended that Henry should succeed to the English throne, as well as to the territories of Normandy, Anjou, and Maine. Richard, who was the favourite of his mother, was to receive her estates of Aquitaine and Poitou; Geoffrey, who had married the daughter of the Duke of Brittany, was to succeed to that province; and John was to be made King of Ireland. During the archbishopric of Thomas Becket, the king had taken measures to show his authority by causing his eldest son to be crowned king by the Archbishop of York. The political enemies of Henry exerted themselves to turn this impolitic measure to their own advantage, by exciting the son to rebellion against the father, who was now called the elder king. In these attempts they were seconded by Queen Eleanor, whose affections had been alienated from the king by his numerous infidelities. She was a woman of strong passions, and determined to make her children the instruments of her vengeance. Through her efforts the people of Aquitaine and Poitou attached themselves to the cause of the younger king, and many of the nobles of those provinces became his counsellors and confidants. They spared no pains to excite the ambition of the youth, and persuade him that his father had abdicated the throne in his favour, and was no longer entitled to hold the sovereign authority. At the coronation of Prince Henry, his wife Margaret, the daughter of Louis of France, was not permitted to receive the crown with her husband, and this omission was resented by the French king, to whom it afforded a pretext for embracing the cause of his son-in-law. A peace having been concluded by the intervention of the Pope, the wrong was repaired, and Margaret was crowned queen. Henry then permitted the young couple to visit the French court, and during their stay, Louis continued to foment the dissatisfaction of the son, and to excite him to rebellion against his father.