Monarch - George V
3 May - The province of Northern Ireland is created within the United Kingdom under terms of the Government of Ireland Act 1920.
13 May - Irish elections, under terms of the Government of Ireland Act 1920: At close of nomination for elections to the new Parliament of Southern Ireland, all 128 candidates are returned unopposed and deemed elected. All 124 Sinn Féin candidates regard themselves as elected to the Second Dáil.
24 May - Irish elections, under terms of the Government of Ireland Act 1920: In the Northern Ireland general election for the new Parliament of Northern Ireland (held by single transferable vote), it is apparent by 29 May that the Unionists have a substantial majority (40 out of 52 seats). The six Sinn Féin members consider themselves elected to sit in the Second Dáil (where five also hold seats representing Southern Ireland).
7 June - The 40 elected Unionist Members of Parliament gather in Belfast City Hall. James Craig is elected as the first Prime Minister of Northern Ireland.
22 June - State opening of the first parliament at Belfast City Hall is conducted by George V of the United Kingdom who makes a speech (drafted by Jan Smuts) calling for reconciliation in Ireland.
4 July - James Craig refuses to attend a peace conference in Dublin because the invitation by President Éamon de Valera was addressed to him personally instead of to the Prime Minister of Northern Ireland.
8 July - At the Peace Conference in the Mansion House, Dublin, President de Valera accepts an invitation to meet the Prime Minister of the United Kingdom, David Lloyd George, in London.
10 July - Bloody Sunday: Clashes between Catholics and Protestants in Belfast result in 16 deaths (23 over the surrounding four-day period) and the destruction of over 200 (mostly Catholic) homes.
12 July - Sinn Féin representatives arrive in London for talks.
18 July - Ulster Unionist negotiators walk out of the truce talks in London.
23 August - The Northern Cabinet agrees that Stormont Castle will be the permanent site of the Northern Houses of Parliament.
11 October - The Irish Treaty Conference opens in London.
21 November - Troops are sent to restore order after rioting breaks out in East Belfast.
22 November - At least ten people die in widespread shootings in Belfast.
6 December - Agreement is reached in the Treaty negotiations in London. The main points of the agreement include the creation of an Irish Free State within the Commonwealth, an Oath of Allegiance to the Crown and the Royal Navy will be able to use certain Free State ports.
16 December - The British House of Commons accepts the Articles of Agreement. The House of Lords also votes to accept the Treaty by a large majority.
International
The new Football Association of Ireland then known as the Free State FA is formed in Dublin after a split from the Belfast-based Irish Football Association.
Irish League
Irish Cup
February - Frank Hall, journalist and satirist (died 1995).
18 February - Brian Faulkner, Baron Faulkner of Downpatrick, sixth and last Prime Minister of Northern Ireland, Ulster Unionist Party MP (died 1977).
11 June - Alexander Faris, light music conductor and theme tune composer (died 2015)
16 June - Sheila McGibbon, actress (died 1997).
25 August - Brian Moore, novelist (died 1999).
26 October - George Forrest, Ulster Unionist MP for Mid Ulster (died 1968).
9 November - Professor John Dundee, anaesthetist (died 1991).
23 October - John Boyd Dunlop, inventor (born 1840)
1921 in Northern Ireland Wikipedia (Text) CC BY-SA
