The so-called Criminal Action Force (CAF) – which claims to comprise of criminals from various gangs who have joined forces to oppose the Real IRA's expansion in Dublin's underworld – does not exist, according to sources.

In recent weeks, a group calling itself CAF issued statements and a photograph to a national newspaper claiming gangs across Dublin had united to hit back at republicans who have extorted more than €400,000 from them so far this year.

They threatened to murder any dissident Republicans who continue to attempt to extort money from criminal gangs. However, sources said there is "absolutely no garda intelligence whatsoever" to support the existence of the group.

"The Criminal Action Force does not exist as far as we're concerned. It's a joke. It is a publicity stunt, plain and simple," said a source. "Criminal gangs are unhappy that dissident Republicans are trying to extort money from them. But they have not joined forces in any way to oppose them."

In recent months, four brothers and their criminal associates in Donaghmede, north Dublin, have been extorting money from crime gangs after establishing themselves as members of the Real IRA.

The dissident republicans have been demanding a cut from profits from other crime gangs who have been involved in armed robberies and, to a lesser extent, have been muscling in on control of security at Dublin bars and nightclubs.

There has also been conflict over the control of drug-dealing in various parts of the city. "The brothers in Donaghmede and their associates have been flexing their muscles, demanding a cut of money from crime gangs if they see a weakness within that gang. This has obviously caused a lot of friction," said the source.

Detectives are closely monitoring the activities of the Real IRA criminals, particularly the brothers in Donaghmede. Two of the brothers were arrested three weeks ago within hours of criminal Sean Winters being shot in the head in an affluent Dublin suburb. Winters had ties to a north Dublin gang involved in drug dealing and is believed to have been resisting an att­empt to hand over money de­manded of him by the Real IRA.