Dublin Castle was until 1922 the seat of the United Kingdom government's administration in Ireland, and is now a major Irish government complex. Most of it dates from the 18th century, though a castle has stood on the site since the days of King John, the first Lord of Ireland. The Castle served as the seat of English, then later British government of Ireland under the Lordship of Ireland (1171–1541), the Kingdom of Ireland (1541–1800), and the United Kingdom of Great Britain and Ireland (1800–1922).

After the signing of the Anglo-Irish Treaty in December 1921, the complex was ceremonially handed over to the newly formed Provisional Government led by Michael Collins.

Dublin Castle fulfilled a number of roles through its history. Originally built as a defensive fortification for the Norman city of Dublin, it later evolved into a royal residence, resided in by the Lord Lieutenant of Ireland or Viceroy of Ireland, the representative of the monarch. The second in command in the Dublin Castle administration, the Chief Secretary for Ireland, also had his offices there. Over the years parliament and law courts met at the castle before moving to new purpose-built venues. It also served as a military garrison. "Castle Catholic" was a pejorative term for Catholics who were considered to be overly friendly with or supportive of the British administration.

Upon formation of the Free State in 1922, the castle assumed for a decade the role of the Four Courts on the Liffey quays which had been badly damaged during the Civil War.

It was decided in 1938 that the inauguration of the first President of Ireland, Douglas Hyde would take place in the castle, and the complex has been host to this ceremony ever since. The castle is also used for hosting official State visits as well as more informal foreign affairs engagements, State banquets, and Government policy launches, as well as acting as the central base for Ireland's hosting of the European Presidency approximately every 10 years.