The Anglo-Norman invasion of Ireland took place during the late 12th century, when Anglo-Normans gradually conquered and acquired large swathes of land from the Irish, over which the kings of England then claimed sovereignty, all allegedly sanctioned by the purported Papal bull Laudabiliter. At the time, Gaelic Ireland was made up of several kingdoms, with a High King claiming lordship over most of the other kings. The Norman invasion was a watershed in Ireland's history, marking the beginning of more than 800 years of direct English/Welsh and, later, British, involvement in Ireland.

In May 1169, Anglo-Norman mercenaries landed in Ireland at the request of Diarmait mac Murchada (Dermot MacMurragh), the deposed King of Leinster, who sought their help in regaining his kingship. They achieved this within weeks and raided neighbouring kingdoms. This military intervention was sanctioned by King Henry II of England. In return, Diarmait had sworn loyalty to Henry and promised land to the Normans.

In 1170 there were further Norman landings, led by the Earl of Pembroke, Richard "Strongbow" de Clare. They seized the important Norse-Irish towns of Dublin and Waterford, and Strongbow married Diarmait's daughter Afe. Diarmait died in May 1171 and Strongbow claimed Leinster, which Diarmait had promised him. Led by High King Ruaidr Ua Conchobair (Rory O'Connor), a coalition of most of the Irish kingdoms besieged Dublin, while Norman-held Waterford and Wexford were also attacked. However, the Normans managed to hold most of their territory.

In October 1171, King Henry landed with a large army to assert control over both the Anglo-Normans and the Irish. This intervention was supported by the Roman Catholic Church, who saw it as a means of ensuring Irish religious reform, and a source of taxes. At the time, Irish marriage laws conflicted with those of the broader Church, and the Gregorian Reform had not been fully implemented. Henry granted Strongbow Leinster as a fiefdom, declared the Norse-Irish towns to be crown land, and arranged the synod of Cashel to reform the Irish church. Many Irish kings also submitted to him, likely in the hope that he would curb Norman expansion, but Henry granted the unconquered kingdom of Meath to Hugh de Lacy. After Henry's departure in 1172, fighting between the Normans and Irish continued.

The 1175 Treaty of Windsor acknowledged Henry as overlord of the conquered territory and Ruaidr as overlord of the remainder of Ireland, with Ruaidr also swearing fealty to Henry. The Treaty soon collapsed: Norman lords continued to invade Irish kingdoms and the Irish continued to attack the Normans. In 1177, Henry adopted a new policy. He declared his son John to be "Lord of Ireland" (i.e. claiming the whole island) and authorised the Norman lords to conquer more land. The territory they held became the Lordship of Ireland, part of the Angevin Empire. The Normans' success has been attributed to military superiority and castle-building, the lack of a unified opposition from the Irish and the support of the church for Henry's intervention.

The Normans (Norman: Normaunds; French: Normands; Latin: Nortmanni/Normanni) were a population arising in the medieval Duchy of Normandy from the intermingling between Norse Viking settlers and indigenous West Franks and Gallo-Romans. The term is also used to denote emigrants from the duchy who conquered other territories such as England and Sicily. The Norse settlements in West Francia followed a series of raids on the French northern coast mainly from Denmark, although some also sailed from Norway and Sweden. Said settlements were finally legitimized when Rollo, a Scandinavian viking leader, agreed to swear fealty to King Charles III of West Francia following the siege of Chartres in 911 AD. The intermingling in Normandy produced an ethnic and cultural "Norman" identity in the first half of the 10th century, an identity which continued to evolve over the centuries.The Norman dynasty had a major political, cultural and military impact on medieval Europe and the Near East. The Normans were historically famed for their martial spirit and eventually for their Catholic piety, becoming exponents of the Catholic orthodoxy of the Romance community. The original Norse settlers adopted the Gallo-Romance language of the Frankish land they settled, with their Old Norman dialect becoming known as Norman, Normaund or Norman French, an important literary language which is still spoken today in parts of mainland Normandy (Cotentinais and Cauchois dialects) and the nearby Channel Islands (Jèrriais and Guernésiais). The Duchy of Normandy, which arose from the Treaty of Saint-Clair-sur-Epte, was a great fief of medieval France. The Norman dukes exercised independent control of their holdings in Normandy, while at the same time being vassals owing fealty to the King of France, and under Richard I of Normandy (byname "Richard sans Peur" meaning "Richard the Fearless") the Duchy was forged into a cohesive and formidable principality in feudal tenure. By the end of his reign in 996, the descendants of the Norse settlers "had become not only Christians but in all essentials Frenchmen. They had adopted the French language, French legal ideas, and French social customs, and had practically merged with the Frankish or Gallic population among whom they lived". Between 1066 and 1204, as a result of the Norman conquest of England, most of the kings of England were also dukes of Normandy. In 1204, Philip II of France seized mainland Normandy by force of arms, having earlier declared the Duchy of Normandy to be forfeit to him. It remained disputed territory until the Treaty of Paris of 1259, when the English sovereign ceded his claim to the Duchy, except for the Channel Islands. In the present day, the Channel Islands (the Bailiwick of Guernsey and the Bailiwick of Jersey) are considered to be officially the last remnants of the Duchy of Normandy, and are not part of the United Kingdom but are instead self-governing Crown Dependencies.The Normans are noted both for their culture, such as their unique Romanesque architecture and musical traditions, and for their significant military accomplishments and innovations. Norman adventurers played a role in founding the Kingdom of Sicily under Roger II after briefly conquering southern Italy and Malta from the Saracens and Byzantines, and an expedition on behalf of their duke, William the Conqueror, led to the Norman conquest of England at the historic Battle of Hastings in 1066. Norman and Anglo-Norman forces contributed to the Iberian Reconquista from the early eleventh to the mid-thirteenth centuries.Norman cultural and military influence spread from these new European centres to the Crusader states of the Near East, where their prince Bohemond I founded the Principality of Antioch in the Levant, to Scotland and Wales in Great Britain, to Ireland, and to the coasts of north Africa and the Canary Islands. The legacy of the Normans persists today through the regional languages and dialects of France, England, Spain, Quebec and Sicily, and also through the various cultural, judicial, and political arrangements they introduced in their conquered territories.