The Treaty of Windsor, a territorial agreement made during the time of the Norman invasion of Ireland, is signed in Windsor, Berkshire on October 6, 1175 by King Henry II of England and the High King of Ireland, Rory O’Connor (Ruaidrí Ua Conchobair). The witnesses are Richard of Ilchester, Bishop of Winchester; Geoffrey Ridel, Bishop of Ely; Lorcán Ua Tuathail, Archbishop of Dublin; William de Mandeville, 3rd Earl of Essex; Justiciar Richard de Luci; Geoffrey de Purtico, Reginald de Courtenea (Courtenay) and three of Henry’s court chaplains.

Under the Treaty, O’Connor recognizes King Henry II as his overlord and agrees to collect tribute for him from all parts of Ireland. Henry agrees that O’Connor can be king of the areas not conquered by the Normans. But O’Connor cannot control the territories of which he is nominally king. Henry and his barons annex further land without consulting O’Connor.

Overall, the agreement leaves O’Connor with a kingdom consisting of Ireland outside the provincial kingdom of Leinster (as it was then), Dublin and a territory from Waterford Dungarvan, as long as he paid tribute to Henry II, and owed fealty to him. All of Ireland is also subject to the new religious provisions of the papal bull Laudabiliter and the Synod of Cashel (1172).

O’Connor is obliged to pay one treated cow hide for every ten cattle. The other “kings and people” of Ireland are to enjoy their lands and liberties so long as they remain faithful to the kings of England, and are obliged to pay their tribute in hides through O’Connor.

The Annals of Tigernach record that: “Cadla Ua Dubthaig came from England from the Son of the Empress, having with him the peace of Ireland, and the kingship thereof, both Foreigner and Gael, to Ruaidhrí Ó Conchobhair, and to every provincial king his province from the king of Ireland, and their tributes to Ruaidhrí.” The Annals also list the ongoing violence in Ireland at the time. The text reveals a misunderstanding of the scope of the treaty and the matters agreed by the two kings that soon prove fatal to the peace of Ireland. Henry sees O’Connor as his subordinate within the feudal system, paying him an annual rent on behalf of all his sub-kings. O’Connor sees himself as the restored High King of Ireland, subject only to a very affordable annual tribute to Henry.

The treaty breaks down very quickly, as O’Connor is unable to prevent Norman knights from carving out new territories on a freelance basis, starting with assaults on Munster and Ulaid in 1177. For his part Henry is by now too distant to suppress them and is preoccupied with events in France. In 1177 he replaces William FitzAldelm with his 10-year-old son Prince John and names him as Lord of Ireland.