Two employees at the passport office have been questioned by gardai over suspected fraud.
The Department of Foreign Affairs said the members of staff were at the centre of a joint investigation with the gardai over irregularities in two issued passports.
"This is now a criminal investigation into passport fraud," the department said.
The two employees work at the passport office in Dublin.
"The Passport Service takes the issue of passport fraud very seriously and in the last number of years has strengthened measures to identify and prevent potential fraud," the department said.
An investigation into the alleged fraud was launched by the passport office in May.
The department said that internal checks revealed irregularities in two passports issued by the office and the Garda National Bureau of Criminal Investigation was called in.
Customer services provided by the passport office will not be affected by the inquiry, the department said.
A 30-year-old man was arrested yesterday in Skerries, north Dublin and released in the early hours of this morning without charge.
A woman was arrested yesterday afternoon in Dublin and remains in custody in Ballymun in the north of the city.
Both were held under section four of the Criminal Justice Act. A file on the man is being prepared for the Director of Public Prosecutions.
Gardai said its investigation was ongoing.
Tanaiste Eamon Gilmore said no Irish passport holders were at risk from the fraud.
"There's no risk to the passports that are held by Irish citizens," he said.
"We have in place in the passport service very thorough anti-fraud measures. I think it's clear from this those measures were working and are working.
"It is unfortunate at any time if there's fraud associated with an Irish passport, it's something we take very seriously."
Searches in relation to the fraud investigation have taken place in the passport office in Molesworth Street, Dublin and in other locations.