The primary law governing nationality of the Republic of Ireland is the Irish Nationality and Citizenship Act, 1956, which came into force on 17 July 1956. Its regulations apply to the entire island of Ireland, including the Republic of Ireland and Northern Ireland, a constituent part of the United Kingdom.

All persons born in the Republic before 1 January 2005 are automatically citizens by birth regardless of the nationalities of their parents. Individuals born in the country since that date receive Irish citizenship at birth if at least one of their parents is an Irish citizen or entitled to be one, a British citizen, a resident with no time limit of stay in either the Republic or Northern Ireland, or a resident who has been domiciled on the island of Ireland for at least three of the preceding four years. Persons born in Northern Ireland are usually entitled to – but not automatically granted – Irish citizenship, largely under the same terms. Foreign nationals may become Irish citizens by naturalisation after meeting a minimum residence requirement, usually five years. The President of Ireland may also grant honorary citizenship, which entails the same rights and duties as normal citizenship, although this is rare.

Ireland as a whole was previously part of the United Kingdom and local residents were British subjects. Although 26 of the 32 Irish counties gained independence in 1922 and Irish citizens no longer hold British nationality, they continue to have favoured status when residing in the United Kingdom; they are considered to have automatic and permanent permission to live in the UK, and are eligible to vote in UK elections and stand for public office there.

Ireland is a member state of the European Union (EU) and all Irish nationals are EU citizens. They enjoy free movement rights in EU and European Free Trade Association (EFTA) countries and may vote in elections to the European Parliament.