Gardai have arrested two Passport Office employees in connection with a long-running investigation into alleged passport fraud.
Officers believe fewer than 10 "irregular" passports were processed and issued officially and that they ended up in the possession of criminals.
None of the passports has been recovered despite searches over the past eight months.
This brings the number of arrests linked to the investigation to six.
Detectives from the national bureau of criminal investigation detained a woman in Dublin city centre at lunchtime yesterday, and she was taken for questioning to Pearse Street station under section 4 of the Criminal Justice Act.
A man was arrested four hours later in the city centre and also taken to Pearse Street. He was still being held last night but the woman was released without charge.
Both are employees of the Passport Office and were arrested for an alleged offence under section 20 of the Passport Act.
The Department of Foreign Affairs said the arrests were part of a joint investigation by the gardai and the Passport Service at its offices in Molesworth Street and Balbriggan.
The investigation was launched initially by the department in May when internal checks established that there had been "irregularities" in two passports, which had been issued officially.
The passports were genuine, but included information that was false and were issued to people who should not have been in possession of them.
After preliminary inquiries by the department, it was decided to alert the gardai in June. This resulted in a search on the Dublin-Meath border the following month when two suspected criminals were detained for questioning and then released.
Follow-up inquiries led to the arrest of two employees of the Passport Office. They were held at Balbriggan and Pearse Street stations before being set free.
Gardai are satisfied that the passports were not issued to members of any dissident terrorist group but they believe the documentation was to have been used to facilitate serious crime.
Further internal checks by the Passport Service last autumn revealed irregularities in the processing of a further three passports and yesterday's arrests are part of the investigation into that discovery.
Foreign Affairs said last night: "The Passport Service takes the issue of passport fraud very seriously and has strengthened measures to identify and prevent potential fraud."
Tanaiste and Foreign Minister Eamon Gilmore said previously that the issue of passport fraud was taken very seriously by the Passport Office and in the last number of years measures had been strengthened to identify and prevent potential fraud.
Files will be prepared by gardai for the Director of Public Prosecutions in connection with all of the six arrests.