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.

Led by High King Ruaidrí Ua Conchobair (Rory O'Conor), a coalition of most of the Irish kingdoms besieged Dublin, while Norman-held Waterford and Wexford were also attacked.

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.

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.

According to historian John Gillingham, after the Norman conquest, an imperialist attitude emerged among England's new French-speaking ruling elite, and they came to view their Celtic neighbours as inferior and barbarous.

Orderic Vitalis' account says De Montgomery used his troops to aid O'Brien in Ireland and hoping to succeed his father-in-law as king, but had to flee after his hosts turned against him.

In Ireland, Mac Lochlainn invaded the Kingdom of Breifne, forced the submission Rory O'Connor, king of Connacht, and in 1161 gave MacMurrough eastern Meath.

[11] For six months in 1165, the fleet of Dublin, which was under the control of Dermot MacMurrough, was used to aid Henry II's forces in an abortive campaign in north Wales.

[9] In 1155 John of Salisbury, Secretary to the Archbishop of Canterbury, and good friends with the recently elected Anglo-Norman Pope Adrian IV, made an "extraordinary intervention" at the Roman Curia.

This hagiography, written within a year of Malachy's death in 1148, depicted the Irish, in fact highly Christianised, in exaggerated terms as barbaric, semi-pagan and in need of reform.

Diarmait refusing to accept his fate sailed from Ireland on the 1 August for Bristol with his daughter Aoife[18] and sought help from Henry II in regaining his kingship.

Having secured their help, he returned to his home territory of Uí Ceinnselaig (Hy Kinsella) in 1167 with one knight, Richard FitzGodebert, and a small number of soldiers.

[21] On 1 May 1169, Robert FitzStephen and Maurice de Prendergast landed at Bannow Bay, on the south coast of County Wexford, with a force of at least 40 knights, 60 men-at-arms and 360 archers.

[27] They also raided the neighbouring kingdom of Ossory, defeating the forces of king Donnchad Mac Gilla Patraic (Donagh MacGillapatrick) in the battle of Achad Úr.

[41] King Henry apparently feared that Strongbow would set up an independent kingdom in Ireland, which could control the Irish Sea and interfere in English affairs.

[53] The synod sought to bring Irish church practices into line with those of England, and new monastic communities and military orders (such as the Templars) were introduced into Ireland.

[54][49] In early 1172, Henry allowed de Lacy to take royal troops into Meath, where they plundered and burned the monastic towns of Fore and Killeigh.

Some English writers – such as William of Canterbury and Ralph Niger – condemned Henry's military intervention, describing it as an unlawful "hostile invasion" and "conquest".

[57] A poem in the Welsh Black Book of Carmarthen describes Henry "crossing the salt sea to invade the peaceful homesteads of Ireland", causing "war and confusion".

Gerald de Barri felt obliged to refute what he called the "vociferous complaints that the kings of England hold Ireland unlawfully".

[61] In late 1173, Diarmait Mac Murchada's son, Domhnall Caomhánach (Donal Cavanagh), attacked Strongbow's forces in Leinster, killing 200 men.

[62] Around the same time, an Irish army from Thomond and Connacht, led by Domnall Ua Briain (Donal O'Brian), forced the Normans out of Kilkenny and destroyed Strongbow's motte-and-bailey castle there.

[69] Contemporary English historian William of Newburgh wrote that "the military commanders left there by him [Henry] for the government of this subjugated province, desirous either of booty or fame, by degrees extended the boundaries allotted to them".

[67] That summer, the forces of Oriel and the Northern Uí Néill, under Cenél nEógain (Kinel Owen), invaded Meath, led by King Mael Sechlainn Mac Lochlainn.

[73][74] Over the following months, the Anglo-Normans invaded the kingdoms of Desmond, Thomond, and Connacht, while John de Courcy continued his conquest of east Ulster.

Elements that appear afterwards include: large-scale hay-making;[75] cultivated pears and cherries;[75] larger white-fleeced breeds of sheep;[75] and the introduction of various animals such as rabbits, perch, pike and carp.

[75] The Normans also instigated the widespread building of castles by aristocrats, a key component of the feudal system they brought to Ireland, and round towers.

Despite a king in this time being seen as symbol of justice and arbiter, Henry II seems to have fostered inter-Norman rivalry, possibly as a means to rein in the power of his subordinates in Ireland so they posed him no threat.

[80] This was exemplified in 1172 by Henry II's granting of the Irish kingdom of Meath to Hugh de Lacy, to counterbalance Strongbow's domain in Leinster.