In 1922 Ireland was split into two nations , Northern Ireland and the Republic of Ireland . Ireland was partitioned between Northern Ireland and Southern Ireland in 1921 under the terms of Lloyd George 's Government of Ireland Act 1920 during the war of independence between Ireland and Britain . At the conclusion of that war on 6 December 1922 , under the terms of the resulting treaty , Northern Ireland provisionally became an autonomous part of the newly independent Irish Free State .