The New IRA is planning a violent campaign aimed at driving migrants out of Ireland, intelligence officials have warned.
Security bosses fear the republican group, which succeeded the Real IRA, has been exploiting "tensions" on both sides of the Irish border.
A rise in migrant crime on the island has fuelled local fury - as seen in June, when the stabbing of Stephen Ogilvie, allegedly by Sudanese asylum seeker Hadi Alodid, sparked riots in Belfast.
Security sources told the Mail last night that the group, which still pursues a "united Ireland," is capitalising on these grievances.
There are concerns the campaign could push a fresh wave of migrants from Ireland into the UK, especially across the Northern Irish land border.
A video appeared last week on social media in which an unidentified man issued threats against migrants on behalf of the IRA.
"They will take control, and they will give an ultimatum, not to politicians but to the illegal migrants who are in the country," he says.
The man claimed the British Army had "respected" the IRA during the Troubles for their effectiveness and "professionalism".
A rise in migrant crime on the island has fuelled local fury
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"It is now going to be time for civil war, and that is exactly what these guys are going to be doing and can you blame them?
"This will be a major wake-up call for the authorities. These guys will show them how it's done and give them 24 hours to leave."
The Real IRA broke away from the Provisional IRA in 1997 and later merged with the vigilante outfit Republican Action Against Drugs in 2012 to form the New IRA.
The group has built links with Scottish nationalist groups in Glasgow, which help promote it through social media.
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PICTURED: British troops on routine patrol with armoured cars in Belfast, 1973. The man claimed the British Army had "respected" the IRA
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In July, 25-year-old law student Isobella Perrie Sullivan was charged with possessing explosives after a bomb was found in a vehicle in the Republic of Ireland.
Irish police intercepted her during an operation focused on dissident republican activity, and a defence solicitor told the court she had been asked to take a bag to Northern Ireland without knowing its contents.
A source told the Mail: "The New IRA have been very active this year in the North in attacks against the Police Service of Northern Ireland, at Lurgan police station and a car bomb at Dunmurry.
"They have two aims: Irish independence and all migrants out.
PICTURED: Tents housing illegal migrants near the Office of International Protection in Dublin
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"We believe it is part of a power struggle going on between factions within the group of the old IRA, the Real and the New IRA, with the new incarnations advocating a campaign of violent intimidation."
A Home Office spokesman said: "We have a close and collaborative working relationship with Ireland on migration and border security.
"This includes a mutual commitment to protect the Common Travel Area from abuse, delivered through joint work to identify and tackle migration trends as they emerge."