Sinn Féin won 124 of 128 seats in Southern Ireland unopposed, while unionists dominated the new Northern Ireland parliament, cementing the six-county partition established under the Government of Ireland Act. The results, held during the Irish War of Independence, set the stage for the Anglo-Irish Treaty later that year and the eventual creation of the Irish Free State alongside a permanently separate Northern Ireland.
The elections were less a normal democratic exercise than a referendum conducted under gunfire. British forces and the IRA were locked in an escalating guerrilla conflict across the south, and Sinn Féin candidates ran not to sit in a British-sanctioned parliament in Dublin but to use the mandate as further proof of Irish self-determination. Almost no one contested their seats; the vote in the south was essentially uncontested.
In the north, the calculus was different. Unionists, wary of being swallowed by a Catholic-majority independent Ireland, voted decisively for parties committed to remaining part of the United Kingdom, giving Ulster Unionists a comfortable majority in the new Belfast parliament opened weeks later by King George V.
The split outcome hardened a line already drawn on paper by Westminster the year before. Within months, negotiations opened that produced the Anglo-Irish Treaty of December 1921, creating the Irish Free State while leaving Northern Ireland's border essentially untouched—a partition whose consequences, including decades of sectarian violence known as the Troubles, would echo for the rest of the century.