The jiffy bag reportedly sent to Waterloo appears to have its sender's address as Bus Eireann, Dublin, but the company said gardaí had not been in touch.
"Bus Eireann are currently not aware of this and we have no further comment," a spokeswoman said.
An Post identified the stamps as its "Love & Wedding" design for greeting cards, wedding invitations and thank you cards.
Earlier: Gardaí and UK police investigate reports explosive devices found in London were sent from Ireland
Gardaí are conducting a high-level security investigation after reports that at least one of the three explosive packages in London were sent from Ireland.
Counter-terrorism police in London are investigating three packages containing explosives found at Heathrow Airport, London City Airport and Waterloo station.
There were no injuries after one of the packages ignited. London Met said: “small improvised explosive devices” were found in the A4 postal bags.
The Met's Counter Terrorism Command said it was treating it as a “linked series” of incidents.
We're treating the incidents as a linked series & keeping an open mind regarding motives. Flights at Heathrow & City Airport have not been effected. Train services at Waterloo Station continue to operate. As a precaution, some DLR services were suspended but now fully operational— Metropolitan Police (@metpoliceuk) March 5, 2019
It has emerged that at least one of the packages was posted in Ireland, with a suspicion that all three were sent from here.
It is believed at least one of them had an Irish post stamp on it.
London Met contacted the Garda Security & Intelligence Section at Garda HQ and an investigation is underway.
A Garda spokesman told the Irish Examiner: “Gardaí are assisting the London Met with inquiries in relation to the ongoing security incident in London."
One man who was among staff outside the Network Rail office said he was the individual who found the package.
Asked about the discovery, he said: "I'm sorry, I've been told I can't talk about it."
Seemingly slightly shaken up, he joked with colleagues and expressed thanks when best wishes were extended.
Sky News correspondent Alistair Bunkall posted a picture to Twitter which he claimed showed the packages sent to Heathrow airport and Waterloo station.
The stamps appeared similar to some issued by the Irish postal service for Valentine's Day 2018, featuring a heart motif and the words "Love Eire N".
One of the images showed a partially-burned package addressed to the airport.
Photo of one of the suspect devices - this one sent to Heathrow. It caught fire: pic.twitter.com/keObgokRVH— Alistair Bunkall (@AliBunkallSKY) March 5, 2019
"Photo of one of the suspect devices - this one sent to Heathrow. It caught fire," he tweeted.
The source of the images was not immediately clear and PA was not able to verify their veracity.
In a statement, Scotland Yard said: "The Met Police Counter Terrorism Command has launched an investigation after three suspicious packages were received at buildings in London today, Tuesday, 5 March.
"The packages - all A4-sized white postal bags containing yellow Jiffy bags - have been assessed by specialist officers to be small improvised explosive devices. These devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened.
"The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives.
"At approximately 9.55am the Met Police received a report of a suspicious package at The Compass Centre, Nelson Road, Hounslow. The package was opened by staff at the building, causing the device to initiate. This resulted in part of the package burning.
"No one was injured. The building was evacuated as a precaution. Specialist officers attended and made the device safe. The building remains closed as enquiries continue. The building is not based at Heathrow Airport, and flights have not been affected by this incident.
"At approximately 11.40am British Transport Police were called to reports of a suspicious package in the post room at Waterloo Station. The package was not opened. Specialist officers attended and made the device safe. No one was injured.
Transport Secretary Chris Grayling: Police are investigating a small number of security alerts. If you’re travelling and see anything suspicious, please report it to the police by calling 0800 789 321 or visit https://t.co/yYx5nnJtbT #ActionCountersTerrorism— Dept for Transport #StayHomeSaveLives (@transportgovuk) March 5, 2019
"The station was not evacuated however cordons are in place at a small area outside the station on Cab Road. Train services are continuing to operate as normal.
"At approximately 12.10pm police were called to a report of a suspicious package at offices at City Aviation House, Royal Docks, Newham. Staff were evacuated from the building as a precaution. The package was not opened and no one was injured. Specialist officers attended and made safe the device. The building has been reopened.
"As a precaution, DLR services to London City Airport were suspended but are now fully operational. Flights to and from the airport have not been affected.
"The Met Police has issued advice to transport hubs across London to be vigilant for and report suspicious packages to police.
"No arrests have been made.
"Enquiries are ongoing."
British Transport Police said specialist teams were called to Waterloo at 11.20am after reports that a suspicious item was found in the post room.
"Rail staff colleagues followed tested procedures when responding to this item and it is important to stress no passengers were put in harm's way," the force said.
"Specialist units from BTP were quickly on scene and were supported by colleagues from the Metropolitan Police Service. A cordon remains in place on Cab Road, outside Waterloo station.
"The Met are now leading the investigation into this incident and have confirmed that it is linked to two other incidents, one at London City Airport and one at The Compass Centre, Nelson Road Hounslow. BTP are actively supporting the Met in this investigation."
The @metpoliceuk Counter Terrorism Command has launched an investigation after three suspicious packages were found at locations in London.— Sadiq Khan (@SadiqKhan) March 5, 2019
Our thanks go to police, security, transport staff and all involved for their swift actions to keep our city safe.https://t.co/Ow7xxILfRs
London City Airport said in a statement: "We can confirm that City Aviation House has reopened and staff have returned safely, following the evacuation of approximately 100 staff at 12.15pm due to the discovery of a small improvised explosive device.
"Specialist police officers dealt with the device, which did not affect flight operations. The airport terminal remained open throughout. Inquiries by the Metropolitan Police are ongoing."
Aviation House is where a number of airport staff work and is a two-minute walk from the passenger terminal.
The building was reopened shortly before 4pm.
Flights were not affected but Docklands Light Railway services did not stop at the airport for around an hour during the investigation.