Is Dublin in the United Kingdom?

No. Dublin is the capital of the Republic of Ireland—a sovereign, independent country distinct from the United Kingdom.

Key distinctions:

• Republic of Ireland: 26 counties, EU member, euro currency, president (ceremonial), Taoiseach (PM)
• Northern Ireland: 6 counties, part of UK, uses GBP, sends MPs to Westminster
• UK: England, Scotland, Wales, Northern Ireland

The island of Ireland contains two jurisdictions. Dublin has been the capital of the independent Irish state since 1922 (Irish Free State) / 1949 (Republic declared). The UK has no sovereignty over Dublin or the Republic.