From the information provided by the Deputy it appears that the person in question was born outside Ireland.
As only an Irish citizen may hold an Irish passport the Passport Office must be given documentary proof of citizenship before issuing a passport. Generally speaking a person is an Irish citizen if born in Ireland or if born abroad to parents at least one of whom was born in Ireland.
When a person born abroad applies for a passport and claims Irish citizenship on the basis of a parent born in Ireland the Passport Office asks for documentary evidence that the person whose Irish birth certificate is presented is the father or mother of the applicant for a passport.
It was formerly the practice, when a person born abroad applied for a passport and claimed Irish citizenship through the birth of a parent in Ireland, for the Passport Office to ask for only the applicant's foreign birth certificate and the Irish birth certificate of his/her parent. However, to counter the possible misuse of Irish birth certificates by persons born abroad, neither of whose parents was born in Ireland, fraudulently to obtain Irish passports, it was decided some years ago also to ask such applicants for their parents marriage certificates as corroboratory evidence. Civil marriage certificates stating age or date of birth are of assistance in establishing the link between a person born in Ireland and his/her child born abroad.
However, when a passport applicant states that his/her parents were not married to each other, the Passport Office has no alternative but to accept birth certificates alone as proof of Irish citizenship.