The government of Northern Ireland is, generally speaking, whatever political body exercises political authority over Northern Ireland.
[1] The new autonomous Northern Ireland was formed from six of the nine counties of Ulster, being four counties with unionist majorities (Antrim, Armagh, Down and Londonderry), and Fermanagh and Tyrone two of the five Ulster counties which had nationalist majorities.
[2] In large part unionists, at least in the north east region, supported its creation while nationalists were opposed.
Subsequently, on 6 December 1922, the island of Ireland became an independent dominion known as the Irish Free State but Northern Ireland immediately exercised its right to opt out of the new Dominion.
A Northern Ireland Executive was created following the signing of the Sunningdale Agreement in 1974, while the current Northern Ireland Executive under the First Minister and Deputy First Minister, was created in the Belfast (Good Friday) Agreement, and has intermittently been in existence from 1999 to the present.