Gardaí say a parcel bomb found at a postal sorting office in Limerick appears to be identical to a series of devices sent to addresses in England and Scotland.
Bomb disposal experts had to be called to the An Post depot in the Dock Road area of Limerick on Friday morning after the discovery of a suspicious package.
Irish Defence Forces later confirmed a “viable improvised explosive device” had been found in a plastic envelope.
Irish Justice Minister Charlie Flanagan said a stamp used was similar to those on letter bombs sent to major transport hubs in London and the University of Glasgow earlier this month.
He added: “This is a despicable act.
“The sending of incendiary devices like this is totally unacceptable. I hope the people responsible are brought to justice.”
The depot had to be evacuated during the security alert.
A group calling itself the IRA, and known to police as the New IRA, has said it sent the parcel bombs to offices at Heathrow and London City airports and to the University of Glasgow.
The group has further claimed that another parcel addressed to a British Army recruitment officer may not have been discovered yet.
Mr Flanagan said the item found in Limerick could be that fifth parcel.
“We have reason to believe this could be the case,” he said.
The packages sent to London and Glasgow were all A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened.
They were posted with Irish stamps and had Dublin as the return address, prompting gardaí to join the investigation.
No-one was injured in any of the incidents.