By 911, the Vikings, or Northmen, had established themselves so powerfully in a large area round the lower Seine that the territory was formally granted to them as the duchy of the Northmen – later to be called Normandy. They achieved a reputation as warriors in their disputes with their neighbours, and in 1016 were invited to southern Italy as mercenaries. By 1030 they had created a principality of their own at Aversa; in 1059 Pope Nicholas II granted them Sicily, Calabria and Apulia. In 1066, William, Duke of Normandy, with only 5,000 men, took the crown of England by force; by 1167, when Dermot MacMurrough, in need of allies to restore him to his kingdom, outlined to Henry II and his barons the profits to be gained from involving themselves in Irish affairs, the Normans had established a strong kingdom in England and were firmly rooted in Scotland and Wales.

On 1 May 1169, Robert FitzStephen, Meiler FitzHenry and Robert de Barry, with thirty knights, sixty other horsemen and 300 archers, ‘the flower of the youth of Wales’, landed at Bannow Bay, in three ships. A contemporary chronicler, Robert Wace, described typical Norman landing tactics. First ashore were the archers, with bow bent and quiver ready, taking up a position on the beach ready for immediate action. Not until a reconnaissance had been completed and the all-clear given would the men-at-arms, all in full equipment, be allowed to disembark with their horses; the cavalry would then ride inland through the screen of archers. Such tactics were typical of the military sophistication of the Normans, which was to bring them to military victory in Ireland despite their inferior numerical strength.

On 2 May arrived Maurice de Prendergast, with ten men-at-arms and about 200 archers in two ships. Figure 12 shows the landing places of the main invading armies and traces the more important invasion routes. It should be emphasized that only an indication of direction can be given, not precise itineraries.

The two Norman armies were joined by Dermot MacMurrough, with 500 men, and together they took Wexford. After resting at Ferns, the invading force, now with the addition of a number of Leinstermen and Norse from Wexford – about 3,000 strong – launched an attack on Ossory that met with success despite a spirited defence. Successful expeditions were launched against Offelan and Omurethy. Another expedition into Ossory resulted in the defeat of MacGillapatrick at Achadh-ur. At this stage Prendergast seems to have lost enthusiasm for the enterprise and he attempted to return to Wales with his men. When prevented from doing so by MacMurrough, he allied himself with the king of Ossory and caused a great deal of trouble to his erstwhile allies before finally leaving the country.

At this stage the High King, Rory O’Connor, showed unexpected strength in marching into Leinster and forcing MacMurrough to submit to him. He contented himself with taking hostages and exacting from MacMurrough a promise to bring no more foreigners into Ireland and to expel his Norman allies once Leinster was finally subdued. This was a promise that MacMurrough had neither the inclination nor the power to keep.

Late in 1169, two more ships arrived in Wexford, bearing Maurice FitzGerald, FitzStephen’s half-brother, with ten knights, thirty mounted retainers and 100 archers. Shortly after this MacMurrough marched towards Dublin and secured the submission of the citizens. Meanwhile FitzStephen led an army to Limerick to attempt an attack on the high king, but was forced back to Leinster.

In May 1170, Raymond FitzGerald (le Gros) arrived with ten knights and seventy archers. Landing at Dundonnell he there defeated the men of Waterford in battle and awaited Strongbow, who arrived in August with 200 knights and 1,000 other troops, accompanied by Maurice de Prendergast, who had been persuaded to return. Until Strongbow’s arrival, the invasion had been only partially successful; his intervention was to lead the Norman forces to military triumph. Richard FitzGilbert de Clare, called earl of Striguil and popularly known as Strongbow, was a leading Welsh baron. It had been his original agreement with MacMurrough in 1168 to lend military support in exchange for the succession to the kingship of Leinster that laid the foundations for the Norman invasion and gave it its Welsh character. Landing at Crook, Strongbow joined forces with le Gros to attack and take Waterford. They then joined MacMurrough, who confirmed the alliance by giving his daughter to Strongbow in marriage. Together, they captured Dublin in September and launched attacks on Meath and on O’Rourke’s territory, as far as Slieve Gory. MacMurrough then retired to his palace at Ferns, where he died the following year, and Strongbow spent the next year consolidating his military position. After MacMurrough’s death the High King besieged Dublin, but was defeated in a surprise attack in September and forced to retire.

By the autumn of 1171 Strongbow was master of Dublin, Waterford and Wexford. He was, however, uncomfortably aware that Henry II was intending to intervene to curb his power. Strongbow had already defied an order from Henry to return from Ireland, and had hoped to stave off his wrath by sending a message of loyalty. When Henry arrived in October at Crook with about 4,000 troops in 400 ships, he had two objectives: first to secure the submission of the Irish leaders and second to impose his authority on his own barons. He was successful in both his aims, securing the submission of many of the Irish kings, including MacCarthy, O’Brien and O’Rourke. Although O’Connor and many of the northern kings refused to submit, the High King’s resistance was to prove temporary. At a council of Irish bishops in Cashel, fealty was sworn to Henry, and the Irish church was pledged to conform to the practices of the English church. Henry restricted Strongbow’s power in two ways. Although he granted him Leinster, he removed the towns of Dublin, Wexford and Waterford from his jurisdiction and his pretensions to power were controlled by the appointment as justiciar of Hugh de Lacy, to whom the king granted Meath.

With these grants the conquest became a political reality. Although the military struggle was to continue for centuries, by 1172 the Normans were so strongly entrenched in the east and south-east of the country that there could no longer be any hope of their eventual expulsion. The next century was to prove to be a time of consolidation.