The Irish justice minister said police are investigating a link between a suspicious package found at a post depot in Ireland, and the four parcel bombs detected in London and Glasgow earlier this month.
Staff working in the An Post sorting office in Limerick were evacuated following the discovery of a suspicious package around 6am on Friday.
An army bomb squad was deployed to the scene and gardai are now investigating the post stamp similarities to the previously uncovered letter bombs.
A group calling itself “the IRA” claimed responsibility for four devices that were sent to the UK in earlier in March, adding that a fifth parcel had not yet been found.
Parcels were found at office blocks new Heathrow Airport and London City Airport, as well as Waterloo station and Glasgow University. No one was injured.
Justice Minister Charlie Flanagan said: “This is a despicable act. The sending of incendiary devices like this is totally unacceptable.
"I hope the people responsible are brought to justice.”
Asked whether there were similarities between the parcels, Mr Flanagan told RTE: “I understand that to be the case with particular reference to the postage stamp that was used.”
Scotland Yard said officers were working with the authorities in Ireland.
Earlier this month, Scotland Yard said a media outlet in Northern Ireland received the claim of responsibility by a recognised IRA code word.
Met Police also said those claiming responsibility indicated five explosive devices were sent - though only four had been recovered at that point.
eputy Assistant Commissioner Dean Haydon, the Met's Senior National Coordinator for Counter Terrorism Policing said: "Whilst it is still early, from the images we've seen and from speaking to our counterparts in the Republic of Ireland, it appears that the package found in Limerick is linked to those sent to London and Glasgow on 5 and 6 March.
"One line of enquiry is that the package found today in Limerick was returned there having never reached the intended recipient in London.
“We will continue to work closely with our counterparts at both An Garda Síochána and Police Scotland to identify those responsible for sending these packages and enquiries are very much ongoing.
"We are also aware that a claim of responsibility made on 11 March referenced there being a fifth package. Whilst that remains a line of enquiry, we continue to keep an open mind on who may be responsible and any potential motivation.
“I must also stress that we continue to urge the public to remain vigilant for any suspicious packages and to report anything suspicious to police.”
Terror detectives from the Met are continuing to investigate the three packages received in London, while Police Scotland is leading the probe into the device at Glasgow University.
No arrests have been made.
The An Post building was evacuated in Limerick before the Explosives Ordnance Disposal team attended the scene.
A Garda spokesman said: “Shortly after 6am, gardai in Henry Street received a report of a suspect package at the An Post sorting office on Dock Road, Co Limerick."