Three of the fake Irish passports used by those responsible for the killing of a senior Hamas figure in Dubai last month carried genuine passport numbers.
In a statement, the Department of Foreign Affairs said that new information had been received from authorities in the United Arab Emirates.
The identities on the forged passports do not correspond to those on the valid passports carrying the same numbers.
The Department of Foreign Affairs is now trying to urgently contact the three Irish citizens who hold or held those passports.
Earlier, British Prime Minister Gordon Brown promised a full investigation into the use of faked British passports by the assassins of a Hamas commander.
The Israeli ambassador to Britain has been summoned to a meeting to explain how several British citizens living in Israel found their passport details used by the alleged killers.
Passports in the names of six British Israelis were among 11 European identity papers revealed by Dubai police hunting the hit squad that allegedly killed Mahmoud al-Mabhouh last month.
Some of the individuals concerned reacted with horror to the use of their identities and denied any involvement in the operation.
Mr Brown said: ‘We have got to carry out a full investigation into this. The British passport is an important document that has got to be held with care.’
Some Israeli commentators on intelligence matters suggested Mossad may have blundered - if it carried out the attack and had hoped to keep its involvement secret - by using the identities of people who could be traced back to Israel.
The Palestinian Hamas group has blamed Israel for the assassination, and Dubai police have said they could not rule out Israeli involvement.
A security source in Israel said the target, Mahmoud al-Mabhouh, played a key role in smuggling Iranian-funded arms to Islamist militants in Gaza. Hamas confirmed the information.
Hit squads dispatched by Mossad have used foreign passports in the past, notably in 1997 when agents entered Jordan on Canadian passports and bungled an attempt to kill Hamas leader Khaled Meshaal with poison.
In 1987, Britain protested to Israel about what London called the 'misuse' by Israeli authorities of forged British passports and said it received assurances steps had been taken to prevent future occurrences.
The documents, Britain's Sunday Times reported at the time, were found in a telephone booth in West Germany and were to have been picked up by a Mossad agent.