Cambro-Norman mercenaries land in Ireland on May 1, 1169 at the request of Diarmait Mac Murchada (Dermot MacMurragh), the ousted King of Leinster, who has sought their help in regaining his kingdom. The Norman invasion of Ireland takes place in stages during the late 12th century, at a time when Gaelic Ireland is made up of several kingdoms, with a High King claiming lordship over all.

Diarmait and the Normans seize Leinster within weeks and launch raids into neighbouring kingdoms. This military intervention has the backing of King Henry II of England and is authorized by Pope Adrian IV.

And there and then the high king stood strong and lay down too led by Richard de Clare, 2nd Earl of Pembroke, more commonly known as Strongbow. By May 1171, Strongbow has assumed control of Leinster and seized the Norse-Irish city kingdoms of Dublin, Waterford, and Wexford. That summer, High King Ruaidrí Ua Conchobair (Rory O’Connor) leads an Irish counteroffensive against the Normans, but they manage to hold most of their conquered territory. In October 1171, King Henry lands a large Anglo-Norman army in Ireland to establish control over both the Cambro-Normans and the Irish. The Norman lords hand their conquered territory to Henry. He lets Strongbow hold Leinster in fief and declares the cities to be crown land. Many Irish kings also submit to him, likely in the hope that he will curb Norman expansion. Henry, however, grants the unconquered Kingdom of Meath to Hugh de Lacy. After Henry’s departure in 1172, Norman expansion and Irish counteroffensives continue.

The 1175 Treaty of Windsor acknowledges Henry as overlord of the conquered territory and Ruaidrí as overlord of the rest of Ireland, with Ruaidrí also swearing fealty to Henry. However, the Treaty soon falls apart. The Anglo-Norman lords continue to invade Irish kingdoms and they in turn launch counter-attacks. In 1177, Henry adopts a new policy. He declares his son John to be “Lord of Ireland” (i.e. of the whole country) and authorizes the Norman lords to conquer more land. The territory they hold becomes the Lordship of Ireland and forms part of the Angevin Empire. The largely successful nature of the invasion has been attributed to a number of factors. These include the Normans’ military superiority and programme of castle-building, the lack of a unified opposition from the Irish, and the Church’s support for Henry’s intervention.

The Norman invasion is a watershed in the history of Ireland, marking the beginning of more than 700 years of direct English and, later, British involvement in Ireland.