Irish police have arrested four people, two of whom are staff members of the Dublin Passport Office, as part of a criminal investigation into passport fraud.

It is understood two 'irregular' passports were processed and ended up in the possession of criminals.

Three men and a woman, including two officials from the Passport Office, were questioned by police, but all have been released without charge.

The Irish Times reports that a 30-year-old man was arrested in Skerries in North Dublin on Tuesday morning and a woman, also 30, was arrested later that afternoon in connection with the investigation.

Minister for Foreign Affairs Eamon Gilmore said the investigation did not pose any risk to Irish passport holders.

Gilmore said, “We have in place in the passport service very thorough anti-fraud measures.

“You can take it from this episode those measures have worked and are working,” he said today.

“We issue something like 600,000 passports each year. It is unfortunate at any time if there is fraud associated with an Irish passport. It is something we take it very seriously and that it why it is being investigated by the gardai,” he added.

The Dublin Passport Office on Molesworth Street launched an investigation in May when checks revealed irregularities in the issuance of two passports.

The Garda National Bureau of Criminal Investigation was informed and a joint investigation began.

In a statement the Department of Foreign Affairs said 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.”

A file is being prepared for the Director of Public Prosecutions, police said.