<P> During this period, the Government of Ireland Act 1920 repealed the previous 1914 Act, and provided for two separate devolved parliaments in Ireland . It defined Northern Ireland as "the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry" and Southern Ireland "so much of Ireland as is not comprised within the said parliamentary counties and boroughs". Section 3 of this Act provided that the parliaments may be united by identical acts of parliament: </P> <P> Sinn Féin did not recognise this act, treating elections to the respective parliaments as a single election to the Second Dáil . While the Parliament of Northern Ireland sat from 1921 to 1972, the Parliament of Southern Ireland was suspended after its first meeting was boycotted by the Sinn Féin members, who comprised 124 of its 128 MPs . A truce in the War of Independence was called in July 1921, followed by negotiations in London between the government of the United Kingdom and plenipotentiaries of Dáil Éireann . On 6 December 1921, they signed the Anglo - Irish Treaty, which was approved by the Dáil in January 1922, leading to the establishment of the Irish Free State the following year, a dominion within the British Empire, from the territory which had been defined as Southern Ireland . Article 11 of the Treaty allowed the Northern Ireland Parliament to opt into the Irish Free State, an option which was not exercised: </P> <P> In Irish republican legitimist theory, the Treaty was illegitimate and could not be approved . According to this theory, the Second Dáil did not dissolve and members of the Republican Government remained as the legitimate government of the Irish Republic declared in 1919 . Adherents to this theory rejected the legitimacy of both the Irish Free State and Northern Ireland . </P> <P> The report of Boundary Commission in 1925 established under the Treaty did not lead to any alteration in the border . </P>

Why northern ireland should be part of the uk