5. c. 65) was an Act passed by the Parliament of the United Kingdom which redistributed the parliamentary constituencies in Ireland for the House of Commons.

It was enacted on the same day as the Representation of the People Act 1918 which extended the franchise throughout the United Kingdom.

The revision was based on a report of a Boundary Commission carried out in 1917 taking population changes in the 1911 census into account.

In its election manifesto, it called for the establishment of an assembly "comprising persons chosen by Irish constituencies as the supreme national authority to speak and act in the name of the Irish people", which would become the revolutionary Dáil Éireann.

The seats designated for Westminster in the 1920 Act would only apply in the six counties of Northern Ireland which remained part of the UK, taking effect at the 1922 general election.