A viable improvised explosive device has been found at the Limerick An Post sorting office.
Police believe it's linked to a number of similar devices sent to locations in the UK earlier this month.
The suspicious package in Limerick was discovered by staff shortly after 6:00am on Friday morning.
More than 100 people were evacuated from the depot.
The discovery was made in the National Returned Letters Centre, which deals with undeliverable mail.
The Army Bomb Disposal Team was called to the scene, and they made the device safe.
They've confirmed the device was viable.
Gardaí have said this parcel "appears to be identical" to others discovered earlier this month in London and Glasgow.
In a statement, Met Police in the UK said it appears the incidents are linked.
Deputy Assistant Commissioner Dean Haydon explained: "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.
London and Glasgow packages
Packages were sent to transport hubs in the British capital, as well as Glasgow University, earlier this month.
Emergency services were called to incidents at Heathrow Airport, London City Airport and Waterloo train station.
The white envelopes - all A4-sized white postal bags containing yellow Jiffy bags - contained an Irish return address as well as Irish stamps.
UK police said the devices appeared "capable of igniting an initially small fire" when opened.
They are also investigating claims that the 'IRA' is responsible for the explosive devices.
There were previous claims that five devices were sent, despite only four having been discovered in the UK.
An Garda Síochána have said they continue to liaise with the UK authorities in relation to these investigations.
Additional reporting: David Raleigh and Stephen McNeice