A car containing an "extremely significant" bomb has been intercepted in the Republic of Ireland as it was being driven across the border for an attack in Northern Ireland, Irish police have said.

The vehicle was stopped during an operation targeting dissident republican activities on Wednesday but further details are just emerging. Two people were arrested.

Garda Commissioner Justin Kelly confirmed on Friday that officers found an explosive device and described it as an "extremely significant seizure".

The vehicle was stopped on the N2 near Carrickmacross, County Monaghan, at about 15:00 local time on Wednesday.

A woman in her 20s, who was the driver of the vehicle, was arrested on Wednesday afternoon and a man in his 40s has since been arrested.

Irish broadcaster RTÉ has reported that gardaí (Irish police) sources said the bomb contained a power unit, a detonator and military grade explosives.

Bomb was 'not just components or pieces'

Gardaí sources also said it was expected to be used as a under-car bomb in an attack in Northern Ireland, according to RTÉ.

Speaking to reporters in County Tipperary, Kelly said: "Any seizure of this nature is really concerning.

"However, we certainly know that there are a small minority of dissident republicans who did not agree with the Good Friday Agreement, they did not put weapons and explosives (out of use), they didn't, you know, go through with the proper process in relation to decommissioning.

"So that is something that we continue to deal with.

"There's a small minority of these individuals, I would describe them as radicalised republicans, and this is a threat that we're going to continue to face for some time."

Kelly said that the bomb was "not just components or pieces".

"This was an explosive device and it would have or could have potentially caused significant damage.

"This is an intelligence-led operation, so we certainly do have information in relation to the individuals involved."

Ireland's Minister for Justice Jim O'Callaghan described the seizure as "very worrying".

"I think it evidences the fact that there is an ongoing threat in respect of dissident republican activity," he said.

The minister added: "I suppose what I'm encouraged about is that the gardaí were able to apprehend the suspect.

"The device has been defused. I want to commend the gardaí for the work they did in ensuring that this didn't become a real threat to the public."

O'Callaghan said it was "obvious" that "a very serious criminal act" would have occurred if police had not intercepted the suspects.

Northern Ireland Secretary Sir Chris Bryant thanked Irish police for their "quick, decisive action".

"Intercepting a dangerous device like this undoubtedly saved lives and kept communities safe on both sides of the border. Strong cross-border security cooperation remains as vital as ever."

PSNI Assistant Chief Constable Davy Beck said he welcomed the "significant seizure and the arrests" made by Irish police, who he said they work with "very closely on a daily basis".

"This underlines our joint commitment to address the threat posed by violent dissident republican groups and keep communities safe.

"We will continue to liaise closely with An Garda Síochána in relation to their investigation, and provide any support as required," he said.

'It's like a throwback to a different era'

There is shock and surprise in Carrickmacross at the discovery of a bomb in a car in the outskirts of the town.

"It is like a throwback to a different era," said one elderly woman as she passed the spot where media have gathered in the town centre.

During the 30 years of the Troubles, mainly in the 1970s and 80s, weaponry and explosives were smuggled over the border from the Republic of Ireland to Northern Ireland.

Carrickmacross is around seven miles from the border, with Cullaville and Crossmaglen the closest villages in Northern Ireland.

"Thank God, the bomb was found before it went off," said one local man.

"There must have been a tip-off that it was coming this way."

Who are dissident republicans?

The term "dissident republicans" describes a range of individuals who do not accept the Good Friday Agreement - the 1998 peace deal which ended the worst of the Troubles in Northern Ireland.

The Provisional IRA - the main armed republican paramilitary group for most of the Troubles - declared a ceasefire in the run up to the agreement and officially ended its violent campaign in 2005.

Dissident republicanism is made up of various groups which broke away from the Provisional IRA in the 1980s, 1990s and 2000s, including the Continuity IRA and New IRA.

The groups are much smaller than the Provisional IRA, although they have access to high-calibre weapons and have used improvised explosive devices and mortars in attacks and attempted attacks.