Partition of Ireland
Beginning on 21 January 1919 with the Soloheadbeg ambush, through the Irish War of Independence, Irish republicans attempted to bring about the secession of Ireland from the United Kingdom of Great Britain and Ireland. Meanwhile, Irish unionistsmost of whom lived in the northeast of the islandwere just as determined to maintain the Union. The British Government, committed to implementing Home Rule, set up a cabinet committee under the chairmanship of southern unionist Walter Long. The Long Committee recommended the establishment of two devolved administrations, dividing the island into two territories: Northern Ireland and Southern Ireland. This was implemented as the Government of Ireland Act 1920. The act entered into force as a "" on 3 May 1921 and provided that Northern Ireland would consist of the six northeastern counties, while the remainder of the island would form Southern Ireland. It was intended that each jurisdiction would be granted home rule but remain within the United Kingdom. The government of Southern Ireland never functioned: the war for independence continued until the two sides agreed a truce in July 1921, ending with the Anglo-Irish Treaty on 6 December 1921.
Ulster
In early medieval Ireland, a branch of the Northern Uí Néill, the Cenél nEógain of the province of Ailech, gradually eroded the territory of the province of Ulaidh until it lay east of the River Bann. The Cenél nEógain would make Tír Eóghain (most of which forms modern County Tyrone) their base. Among the High Kings of Ireland were Áed Findliath (died 879), Niall Glúndub (died 919), and Domnall ua Néill (died 980), all of the Cenél nEógain. The province of Ulaidh would survive restricted to the east of modern Ulster until the Norman invasion in the late 12th century. It would only once more become a province of Ireland in the mid-14th century after the collapse of the Norman Earldom of Ulster, when the O'Neills who had come to dominate the Northern Uí Néill stepped into the power vacuum and staked a claim for the first time the title of "king of Ulster" along with the Red Hand of Ulster symbol. It was then that the provinces of Ailech, Airgialla, and Ulaidh would all merge largely into what would become the modern province of Ulster.
Domnall Ua Lochlainn (died 1121) and Muirchertach Mac Lochlainn (died 1166) were of this dynasty. The Meic Lochlainn were in 1241 overthrown by their kin, the clan Ó Néill (see O'Neill dynasty). The Ó Néill's were from then on established as Ulster's most powerful Gaelic family.