The United Kingdom of Great Britain and Ireland was formed in 1801 from the merger of the Kingdom of Great Britain (itself a merger of the Kingdoms of Scotland and England in 1707) and the Kingdom of Ireland. The merger was facilitated by the decision of the Irish Parliament in College Green, Dublin in August 1800 to vote itself out of existence by passing the Act of Union. Whilst the Irish Free State became independent in 1922 the United Kingdom of Great Britain and Ireland continued in name however until 1927 when it was renamed as the United Kingdom of Great Britain and Northern Ireland in accordance with the Royal and Parliamentary Titles Act.

Under the terms of the merger, Ireland was to be represented by over 100 MPs in the united parliament, meeting in the Palace of Westminster. Part of the trade-off was to be the granting of Catholic Emancipation. However this was blocked by King George III who argued that emancipating Roman Catholics would breach his Coronation Oath .

Generations of Irish leaders campaigned to establish home government in Ireland. Daniel O'Connell successfully forced the British Government to grant Catholic Emancipation finally in 1829. However his campaign to 'Repeal' the Act of Union failed. Later leaders such as Charles Stewart Parnell campaigned for a version of Irish self government called Home Rule within membership of the United Kingdom.

In 1919, Irish MPs elected to Westminster formed an independent, illegal Irish parliament, Dáil Éireann with an executive under the President of Dáil Éireann, Eamon de Valera. A War of Independence was fought between 1919 and 1921. Finally in December 1922, twenty-six of Ireland's counties exited from the United Kingdom of Great Britain and Ireland and formed an independent Irish Free State. Six counties, called Northern Ireland, remained in the United Kingdom, which was renamed the United Kingdom of Great Britain and Northern Ireland in 1927, as part of a fundamental revision of royal, state and dominion titles. As part of the change, the King ceased to be monarch in the various dominions (each of which was seen as inferior to the United Kingdom) and became king of each dominion separately, becoming King of Ireland, King of Australia , King of Canada, King of New Zealand , etc., replacing the concept of a shared crown with a shared monarch wearing separate crowns.

The Union Flag

The flag created by the merger of the Kingdoms of Great Britain and Ireland in 1801 still remains the flag of the current United Kingdom. Known as the Union Flag or the Union Jack, it combines the then flags of England and Scotland with St. Patrick's flag from Ireland. The red cross, St. George's Cross, represents England. The Blue background, on which St. Andrew's Saltire (in the shape of a white 'x') appears, repesents Scotland, while the red 'x' which overlays the white x' on the blue background of Scotland, is known as 'St. Patrick's Cross' and represents Ireland.