Fitz-Stephen, on returning to Wales, had helped Diarmait Mac Murchada to organize a mercenary army of Norman and Welsh soldiers, including Richard de Clare, second Earl of Pembroke, alias Strongbow.

The first Norman knight to land in Ireland had been Richard fitz Godbert de Roche in 1167, but it is not until May 1, 1169, that Robert leads the vanguard of Diarmait Mac Murchada's Cambro-Norman auxiliaries to Ireland, thereby precipitating the Norman invasion of Ireland.

The main invasion party lands near Bannow strand, County Wexford with a force of thirty knights, sixty man-at-arms and three hundred archers.

The next day, Maurice de Prendergast lands at the same bay with ten knights and sixty archers.

This force merges with about five hundred soldiers commanded by Diarmait.

In return for capturing Wexford, MacMurrough grants Fitz-Stephen a share in two cantreds, Bargy and Forth which comprises all the land between Bannow and the town of Wexford.

The cantreds are to be held jointly with Maurice FitzGerald, Lord of Lanstephan, his half-brother.

The Siege of Wexford lasts only two days.

The first attack is repulsed at the loss of eighteen Normans and three defenders.

These are believed to have been the only deaths during the siege.

Fitz-Stephen then orders his men to burn all the ships in the town's harbor.

The next morning, the attack on Wexford begins again.

Shortly afterward, the defenders send envoys to Diarmait.

The defenders agree to surrender and renew their allegiance to Diarmait.

It is claimed that they were persuaded to surrender by two bishops who were in the town at the time.

He is accompanied at the siege by Robert de Barry, the eldest son of his half-sister Angharad de Windsor. (Nest then, is the mother of Robert, Maurice and Angharad.)

Within a short time, Leinster has been conquered, and the Viking-established towns of Wexford, …

…Dublin are under Diarmait's control.

Henry has allowed the expedition of barons from South Wales to establish Anglo-Norman supremacy in Leinster, but will later levy a fine of one hundred shillings on one Joyce of Gloucester for lending money to Strongbow for his expedition.

Joyce had done nothing illegal, but Henry is nervous about the use of Jewish loans to finance any independent actions or policies.

Rhys benefits in 1169 and 1170 from the Norman invasion of Ireland, which is largely led by the Cambro-Norman lords of south Wales.

The departure of the Norman lords enables Rhys to strengthen his position, and the death of Owain Gwynedd in late 1170 leaves him as the acknowledged leader of the Welsh princes.

Strongbow himself arrives in August 1170.

He soon marries Diarmait's daughter, Aoife, and is named as heir to the Kingdom of Leinster.

This latter development causes consternation to Henry II, who fears the establishment of a rival Norman state in Ireland.

Accordingly, he resolves to visit Leinster to establish his authority.

Diarmuid Mac Murchada dies in May 1171, and his son, Donal MacMurrough-Kavanagh claims the kingdom of Leinster in accordance with his rights under the Brehon Laws.

The Earl of Striguil also claims the kingship in the right of his wife.

The old king's death is the signal of a general rising, and Richard barely manages to keep Rory O'Connor out of Dublin.

At this time, Strongbow sends his uncle, Hervey de Montmorency, on an embassy to Henry.

This is necessary to appease the King, who is growing restive at the Earl's increasing power.

Upon his return, de Montmorency conveys the King's terms—the return of Strongbow's lands in Normandy, England and Wales as well as leaving him in possession of his Irish lands.

De Clare in return surrenders Dublin, Waterford and other fortresses to the King.

Henry lands with a large fleet at Waterford in 1171, becoming the first King of England to set foot on Irish soil.

This marks the beginning of English and later British rule in Ireland.

Both Waterford and Dublin are proclaimed Royal Cities.

Henry accepts the submission of the Irish kings in Dublin in November.

Henry II has taken advantage of an earlier papal bull, “Laudabiliter,” attributed to the late English Pope Adrian IV and authorizing Henry to make himself overlord of Ireland in order to bring the Irish church more into line with Roman standards. (Adrian, who died in 1159, was once believed to have issued the bull, but this document is now assumed to be a forgery.)

The Norman invasion of Ireland thus has the backing of the Papacy.

Henry in 1172 arranges for the Irish bishops to attend the Synod of Cashel, which seeks to regulate some affairs of the Church in Ireland and to condemn some abuses, bringing the Church more into alignment with the Roman Rite.

Adrian's successor, Pope Alexander III, now ratifies the grant of Ireland to Henry, "... following in the footsteps of the late venerable Pope Adrian, and in expectation also of seeing the fruits of our own earnest wishes on this head, ratify and confirm the permission of the said Pope granted you in reference to the dominion of the kingdom of Ireland."

Henry is happily acknowledged by most of the Irish Kings, who see in him a chance to curb the expansion of both Leinster and the Normans.

He now has to leave for England to deal with papal legates investigating the death of Thomas Becket.

Strongbow also agrees to assist the King in his coming war in France.