The principal legislation governing Irish citizenship is the series of Irish Nationality and Citizenship Acts. They are:
The Department of Justice and Equality has published an informal consolidation of the Citizenship Acts available here (pdf).
The Civil Law (Miscellaneous Provisions) Act 2011 (section 33) makes provision for civil partners to receive treatment equal to married couples in the context of citizenship matters. It also provides a statutory framework within which granting of certificates of naturalisation can take place in formal citizenship ceremonies.
Articles 2 and 9 of Bunreacht na hÉireann (the Irish Constitution) (pdf) both deal with citizenship.
Article 2, which became part of the Constitution on 2 December 1999 as a result of the British-Irish Agreement of 1998 (also known as the Good Friday Agreement, the Northern Ireland peace agreement or the Belfast Agreement) states:
"It is the entitlement and birthright of every person born in the island of Ireland, which includes its islands and seas, to be part of the Irish nation. That is also the entitlement of all persons otherwise qualified in accordance with law to be citizens of Ireland. Furthermore, the Irish nation cherishes its special affinity with people of Irish ancestry living abroad who share its cultural identity and heritage."
The Irish Nationality and Citizenship Act 2001 gives effect in Irish citizenship law to Article 1(vi) of the Agreement. In that provision, both the British and Irish Governments recognise "the birthright of all the people of Northern Ireland to identify themselves and be accepted as Irish or British, or both, as they may so choose".
In this context, the term "the people of Northern Ireland" means "all persons born in Northern Ireland and having, at the time of their birth, at least one parent who is a British citizen, an Irish citizen or is otherwise entitled to reside in Northern Ireland without any restriction on their period of residence".
Article 9.2 of Bunreacht na hÉireann, which was inserted by referendum in June 2004, states:
"9.2.1 Notwithstanding any other provision of this Constitution, a person born in the island of Ireland, which includes its islands and seas, who does not have, at the time of the birth of that person, at least one parent who is an Irish citizen or entitled to be an Irish citizen is not entitled to Irish citizenship or nationality, unless provided for by law.
9.2.2 This section shall not apply to persons born before the date of the enactment of this section."
The Irish Nationality and Citizenship Act 2004 specifies the residence requirements that non-national parents of children born in Ireland since 1 January 2005 must fulfil in order for the child to be entitled to Irish citizenship.
If you have a question relating to this topic you can contact the Citizens Information Phone Service on 0761 07 4000 (Monday to Friday, 9am to 8pm) or you can visit your local Citizens Information Centre.