It is much more menacing than the two earlier rebel groupings, the Continuity IRA and the Irish National Liberation Army, which have been intermittently attempting to sabotage the peace process since it began with the first IRA ceasefire in 1994.
They were regarded as a containable fringe of extremists, but in recent months the Real IRA has mounted a substantial terrorist campaign - and has now been joined by the two smaller groups.
It was set up last October by the IRA's former quartermaster, a man from Co Louth in the Irish Republic, who resigned in disgust after the second IRA ceasefire and took with him much of the IRA's bomb-making expertise and access to timing devices, detonators and substantial quantities of explosives. He has now been joined by perhaps 100 IRA activists who feel that the Good Friday peace agreement is a sell-out because it does not lead directly to a united Ireland.
The group is believed to have been responsible for a series of recent attacks, including a 500lb car bomb that devastated the market town of Banbridge in Co Down.
Earlier in the month, a Real IRA bombing campaign in London is believed to have been foiled with a series of arrests, and several Real IRA car bombs destined for England or Ulster are understood to have been intercepted by the Irish police this year.
In May, after a mortar attack on a police station in Co Fermanagh, the group declared that it had appointed an "army executive" and that a "war machine is once again being directed at the British Cabinet".
Two names were yesterday on the lips of everyone in Ireland discussing the republican dissident groups: Bernadette Sands and Michael McKevitt.
Ms Sands, sister of Bobby Sands, the IRA hunger striker who died in the Maze Prison in 1981, is an unflinching republican extremist who last year founded the 32-County Sovereignty Movement, regarded by the security forces on both sides of the border as the Real IRA's political face.
Michael McKevitt, a well-known republican shopkeeper from Dundalk, who was shot in both legs in 1975 in a feud with the Official IRA, is her partner.
According to Ms Sands-McKevitt,as she styles herself, she is simply a spokesperson for a movement she says has no associations with terrorism and is not even a political party, but rather a pressure group for Irish independence.
Ronnie Flanagan, Chief Constable of the Royal Ulster Constabulary, said some months ago he believed the head of the Real IRA was aligned to the Sovereignty Movement, or Sovereignty Committee as it was then called.
He added: "People very close to that committee have a military capability, have an expertise and knowledge that we would assess has been deployed in recent attacks. There is a knowledge and expertise there that could be brought to bear in terrorist attacks."
Last night, the 32-County Sovereignty Movement said the killing of innocent people could not be justified. "We are deeply saddened and devastated by the terrible tragedy in Omagh," it said in a statement.
"The killing of innocent people cannot be justified in any circumstances. We are a political movement and are not a military group. We reject categorically suggestions which have been publicly made that our movement was responsible in any way."
Sinn Fein's national chairman, Mitchel McLaughlin, said last night police on both sides of the border knew who were responsible. "We are talking here about a relatively small number of people who in all probability are guilty of what happened yesterday," he said.
"The unfortunate reality is that there are those, possibly very young and inexperienced and idealistic people, who are being affected by the rhetoric of these people who we don't know."
Earlier this year, Ms Sands-McKevitt conducted a newspaper interview in mourning clothes, having attended the funeral of a Real IRA member.
She commands instant attention in Ireland as the sister of Bobby Sands. The hunger striker, who was elected to Westminster while in the Maze, remains one of the republican movement's most potent icons and martyrs and a large portrait of him is the most prominent republican mural on the Falls Road in Belfast.
His sister's argument is essentially that he did not die, after a 66- day hunger strike, for the sort of agreement Sinn Fein has just accepted. "I can't see what they are doing as being compatible with what Bobby died for," she said.
A mother of three children, she runs a small business, and was largely unknown to the public until last year, when she stepped out from a background role to give media interviews.
She often forecasts republican violence will continue, though her predictions are cast not in the form of threats but as statements of the inevitable.
She and her movement are implacable opponents of the peace process and the Good Friday agreement. She told one interviewer: "The whole thing is just a restructuring of British rule in Ireland."
Criticising Sinn Fein leaders Gerry Adams and Martin McGuinness, she said: "False hope was generated by some people. The message sent out was that there was light at the end of the tunnel for republicans and, if we held hands, we would come through. But that wasn't true. I am firmly convinced we are being conned."Reuse content