As only limited details have been supplied about the individual in question, it is not possible to provide specific guidance on this case. I can, however, provide some general advice in relation to establishing eligibility to hold an Irish passport.
All passport applications are subject to the provisions of the Passports Act 2008. The Act provides, among other things, that a person must be an Irish citizen before a passport can be issued to him or her. In order to meet this requirement, each person must demonstrate an entitlement to Irish citizenship by providing acceptable documentary evidence of this entitlement.
Entitlement to Irish citizenship is determined by the Irish Nationality and Citizenship Act 1956, as amended, under which Irish citizenship may be obtained by birth, by descent, or by naturalisation.
An individual born on the island of Ireland before 2005 is automatically an Irish citizen. For individuals born outside of Ireland, they may claim citizenship if they had at least one parent who was born in the island of Ireland before 2005.
The Passport Service would be happy to provide further information and guidance to the person in question. The Passport Service operates a Customer Service Centre which provides information via telephone and webchat. The telephone number for the Customer Service Centre is 01 671 1633 and webchat can be accessed through the passport service's website. Telephone lines operate from 9 till 5 Monday to Friday (excluding bank holidays) and webchat is available from 9am to 4pm, Monday to Friday (excluding bank holidays).