Mr Mabhouh was electrocuted and then strangled, reports said
Irish passports used by three people believed to have been involved in killing a Hamas member had genuine numbers, the Irish government has said.
Mahmoud al-Mabhouh was murdered in his hotel room in Dubai on 20 January.
The Republic's Department of Foreign Affairs said the numbers on the passports corresponded to those on three real Irish passports.
However, the people identified in the passports recorded in Dubai were not those in the genuine passports.
Previously the department said the passport numbers were counterfeits, as they had the wrong number of digits and had no letters. It said it had since been given "additional material relating to the passport numbers".
The Department of Foreign Affairs said it was trying to contact the three Irish citizens who hold or have held passports containing these numbers.
Irish Foreign Affairs Minister Micháel Martin said: "Our passport is widely regarded and respected throughout the world as being of the highest quality. We have invested very heavily in extra security features so that our citizens can travel in safety.
"Actions, which endanger our well earned reputation in this area, have the potential to affect the security of all our citizens travelling overseas. I am determined to maintain the good name of Irish passports.
"The department is liaising closely with the UAE authorities and with the Garda (Irish police) on the matter."
The department has reportedly summoned the Israeli ambassador to discuss the issue.
After the murder of the Hamas man, police in Dubai issued arrest warrants for 11 "agents with European passports".
The city's police chief said six of the suspects had British passports, three were Irish, one French and one German.
The three Irish suspects were identified as Gail Folliard, Evan Dennings and Kevin Daveron.
Officials in Dubai said the team appeared to be a professional hit-squad, most likely sponsored by a foreign power, suggesting the team were operating on false documents.
Mr al-Mabhouh was a founder member of the Izzedine al-Qassam Brigades, and was thought to be behind the kidnap and murder of two Israeli soldiers in 1989 during the first Palestinian Intifada.
The Izzedine al-Qassam Brigades have been responsible for suicide bombings and rocket attacks across Israel.
Israel has refused to comment on the accusations its security forces were behind the killing.