The Government of Ireland Act 1920, an Act of the Parliament of the United Kingdom also known as the Fourth Home Rule Bill, becomes law on December 23, 1920.

The Act is intended to establish separate Home Rule institutions within two new territories of Ireland – the six northeastern counties of Antrim, Armagh, Down, Fermanagh, Londonderry, and Tyrone are to form “Northern Ireland,” while the remaining 26 counties of the country are to form “Southern Ireland.” Each territory is to be self-governing, except in areas specifically reserved to the Parliament of the United Kingdom such as defence, foreign affairs, international trade, and currency. Provision is made for their future reunification under common Home Rule institutions.

Home Rule never takes effect in Southern Ireland due to the Irish War of Independence, which results instead in the Anglo-Irish Treaty and the establishment in 1922 of the Irish Free State. However, the institutions set up under this Act for Northern Ireland continue to function until they are suspended by the British parliament in 1972 as a consequence of the Troubles.

The final provisions of the 1920 Act remaining in force are repealed under the terms of the Northern Ireland Act 1998, after the Good Friday Agreement. In the republic, the Statute Law Revision Act 2007 repeals the Act almost 85 years after Constitution of the Irish Free State replaced it as the basic constitutional law.