The general rules in relation to the re-acquisition of Irish citizenship differ depending on whether the person in question has a constitutional entitlement to Irish citizenship. Where a person has a constitutional entitlement to citizenship, such entitlement subsists even where the person renounces citizenship — in other words even though the person renounces citizenship he or she remains constitutionally entitled to it. In practice, such a person can exercise that subsisting entitlement by declaring his or her citizenship in the prescribed manner.
In all other cases it is for the Oireachtas to determine on rules relating to the re-acquisition of citizenship. The position as enunciated in section 21 of the Irish Nationality and Citizenship Act 1956, which covered all Irish citizens at the time of its enactment, was as follows. An Irish citizen of full age, who was a citizen of another country and for that reason desired to renounce citizenship, could do so by lodging a declaration of alienage. A condition precedent was that the person in question was ordinarily resident outside the State. Such a person was required to apply for naturalisation if he or she wished to re-acquire citizenship.
Subject to one exception, that remains the position in relation to all former Irish citizens who do not have a constitutional entitlement to Irish citizenship. This includes persons who acquired citizenship by descent alone, by post-nuptial declaration and through the naturalisation process. It also includes persons born in the island of Ireland on or after 24 June 2004 who, at the time of birth, do not have at least one parent who is or is entitled to be an Irish citizen. The one exception arises in this type of case. Such persons, if born between 24 June 2004 and date of implementation of the relevant provisions of the Irish Nationality and Citizenship Act 2004, 1 January 2005, continue to be dealt with for the purposes of re-acquisition after renunciation as if they are constitutionally entitled to citizenship. This exception is simply a reflection of the passage of time between the enactment of the constitutional amendment and the implementation of the primary legislation and the prospective approach taken in matters of this nature.