Specialist gardai are examining a fifth "love bomb" addressed to a major London transport hub after it was discovered in an sorting office here.
The envelope containing the viable explosive was discovered in the National Return Letter Centre in Limerick at around 6am yesterday and led to an immediate evacuation of the building.
It had a return address for Dublin and has been described as being "identical" to other letter bombs sent to the UK earlier this month.
It was later defused by the Army's bomb squad and handed over to gardai for analysis.
The New IRA claimed responsibility for the letter bombs earlier this month after contacting a newspaper and giving a recognised code word.
The dissident group said that in total five devices had been sent, but only four were discovered in England and Scotland. Gardai believe the letter bomb found yesterday is the fifth explosive sent.
Counter-terrorism gardai are continuing to liaise with their colleagues in the UK in an effort to identify those behind the letter bomb campaign.
The dissident group is being treated as the main suspect for the series of postal explosives but gardai have stressed that other lines of inquiry are also being explored.
The envelope was addressed to Charing Cross railway station, near London's Trafalgar Square, but was returned to Ireland by the Royal Mail.
The return address given was Ireland Tourism Dublin but it was sent back to the postal centre in Limerick where undeliverable mail is sent.
It was returned on March 5 - the same day that explosive devices were found at London City and Heathrow airports, as well as Waterloo railway station. The following day a similar bomb arrived at the University of Glasgow.
None of the devices exploded, but the device sent to Heathrow briefly ignited, although no injuries were reported.
A senior source last night told the Herald that detailed analysis of the device would be carried out by gardai to recover evidence which could identify a suspect.
"It will obviously go through all the relevant tests. We will look for DNA, fingerprints and gardai will also be examining the components of the bomb to see where they came from," the source said.
"How much incendiary was in the bomb has not yet been determined but a real danger would have been it catching fire in a mail room and spreading through other envelopes and letters, which would have caused serious damage and risk to workers in postal centres that this envelope passed through."
Detectives have not yet been able to establish the exact location from where the letter bombs were sent, but are satisfied they were posted within Ireland.
One of the letter bombs sent to the UK had a return address for Bus Eireann in Dublin.
The investigation involves detectives based at Henry Street Garda Station, who are being assisted by a number of garda national units including the Special Detective Unit (SDU) and members of the Security and Intelligence Section.
The Defence Forces said that its Army Bomb Disposal Team was requested to the An Post centre on Dock Road, Limerick, by gardai and arrived at 9.50am.
"On arrival, a cordon was established and the building was evacuated for the duration of the operation," it said.
"A viable improvised explosive device contained in a plastic envelope was identified and made safe. The device was then handed over to An Garda Siochana for further investigation and the scene was declared safe.
"Should members of the public encounter suspicious items, or hazardous substances, they are advised to maintain a safe distance and inform An Garda Siochana."
Justice Minister Charlie Flanagan said the sending of a suspected letter bomb was "an absolutely despicable act".
Mr Flanagan also linked the finding of the suspect package in Limerick to the discovery in the UK of explosive devices posted in letters.
He said that the incident was "a dreadful consequence of the uncertainty surrounding Brexit where in the context of the heightened tensions in Northern Ireland, on the island of Ireland, people have taken it upon themselves to send letter bombs - an absolutely despicable act".
Mr Flanagan noted that four similar letters arrived in Britain around two weeks ago and intelligence had suggested that there had been five.
"This could well be the fifth. As yet it's unconfirmed but I understand gardai are actively engaged on the ground in Limerick," he said.
He told RTE Radio's Today With Sean O'Rourke that he has reason to believe it was linked to those in the UK.
"I have to say that the sending of incendiary devices or letter bombs like that is a totally unacceptable act and I would hope that those responsible could certainly be brought to justice," Mr Flanagan added.
"I know that gardai are actively engaged as we speak."
Gardai said that the parcel appeared "to be identical to parcels discovered earlier this month in London and Glasgow".
A spokesman for An Post said: "With the exception of some parcel dispatches, Limerick mail deliveries were disrupted by events at An Post Dock Road depot this morning."
The New IRA has already carried out a car bomb attack outside a courthouse in Derry city in January.
The faction is regarded by An Garda Siochana and the PSNI as the most dangerous terrorist group since the Provisional IRA declared a ceasefire more than two decades ago.
The terror group has more than 50 activists and another 200 supporters, and some have acquired counter-surveillance skills by attending courses in countries such as the Czech Republic and Slovakia.