Dissident republicans linked to the groups which carried out the murders of two soldiers and a policeman in Northern Ireland last month were responsible for the break-in and criminal damage of the Shell gas plant in Mayo last week.

The group of around 18 men wearing black balaclavas threatened and struck one of the two security guards on duty with an iron bar before causing damage costing thousands of euro in what Garda sources described as a "military-style" operation.

They used bolt cutters to break into the compound before commandeering a mechanical digger and destroying fencing and the entrance gates. A delivery lorry was also heavily vandalised at the scene.

When additional security staff arrived they group withdrew at around 12.30am. A cache of improvised weapons; metal bars, a sledgehammer and wooden fence posts with nails, were recovered later.

Gardai disputed the claims by a local protest leader, Willie Corduff, that he was injured in a separate incident after he had been hiding under the trailer of a lorry at the plant.

Gardai said Mr Corduff had complained of head and chest pains necessitating an ambulance and two paramedics to travel out from Castlebar. They examined and found no injuries, according to gardai.

There is no CCTV footage of an assault and no assault is being investigated by gardai. Mr Corduff had also made no complaint to gardai by yesterday. He spent much of Wednesday under the trailer and was eventually removed at around 4am.

The protest has been attracting dissident republicans from both sides of the Border. "They are using it as a recruiting ground," said one senior Garda source.

Last year gardai arrested one man during a violent protest who was subsequently questioned about the murders of the two soldiers in Antrim.

Another northern republican detected at the scene was previously a member of the Provisional IRA and was serving a 20-year sentence for bombing when he was released under the Good Friday Agreement.

Another ex-IRA prisoner now associated with the group Eirigi, which is playing an active role in the protest, was also arrested at the plant last year but not charged.

When the gas is running it will employ 130 people at the plant with a further 55 staff jobs in Mayo. The protesters refused to submit to the arbitration process set up by the Government but do attend the "forum" meetings under the former Department of Justice general secretary, Joe Brosnan.

A widely supported local group, which is in favour of the building of the gas terminal and pipeline, condemned the attack.

Pro Gas Mayo said: "People who try and justify this type of behaviour in some sections of the media claim they are concerned about proper planning, permits and such matters. That being so it is ironic that they cloak their law breaking with such alleged concerns.

"What no one can do is take the law into their own hands. On occasions protesters will use the court system when it suits them, yet they can abuse the court system at other times as happened at last court sitting in Belmullet when the court had to be adjourned three times.

"They do not speak for the vast majority of decent people in Erris and Mayo, where so many people now find work at a time of recession and we hope the authorities will deal with such law breaking in a fast and efficient manner, so that the project can be finished as soon as possible."

Eirigi, the dissident republican group which has been involved in the protesting for the past 18 months, condemned what it terms the "26 county state" for allowing the Corrib plant to go ahead.