A murder investigation on both sides of the Irish border will decide the fate of Northern Ireland’s fragile power-sharing experiment, after Ian Paisley’s Democratic Unionists said that IRA involvement could mean the collapse of the political institutions.

The killing of Paul Quinn, a 21-year-old lorry driver who was beaten by a gang wielding iron bars and subsequently died on Saturday, is the first test of the relationship between the DUP and Sinn Fein, who share power in the Northern Ireland Executive.

Gerry Adams, the Sinn Fein leader, yesterday categorically ruled out any republican or Provisional IRA involvement in the murder. But the dead man’s family say that their son was killed by Provisionals because he had ignored an order by the terrorist group to leave the country.

The manner in which Mr Quinn was lured to his death and killed also bore the hallmarks of the Provisional IRA, in particular those of the local South Armagh brigade.

There will be pressure on the Police Service of Northern Ireland (PSNI) and the Irish police to establish quickly who was responsible. Jeffrey Donaldson, MP for Lagan Valley and a member of the DUP, said that if it was found that the IRA was involved there would be serious repercussions.

“If the IRA were corporately involved in this murder, that may mean that the Executive is finished in its current form. But we do not yet know and we have to wait until we get all of the facts,” he said.

Mr Donaldson qualified his use of the word “corporate” when he was challenged by Jim Allister, a former party colleague and MEP who left the DUP when it agreed to share power with Sinn Fein. Mr Allister said the use of the word suggested that efforts were under way to distance the IRA from the actions of individual members to give the DUP a lifeline to continue in government with Sinn Fein.

Mr Donaldson added that if any IRA member had been involved in the attack on Mr Quinn, he would consider it to be an IRA action.

Mr Quinn was killed after two friends, who were working on a farm, were assaulted, tied up and forced to telephone him. When he arrived from his home in Crossmaglen, South Armagh, he was set upon by some 15 men wielding iron bars. He died a short time later.

Jim McAllister, a family friend of the Quinns and a former Sinn Fein councillor, said: “The beating was overheard by the lads in the sheds. There were horrifying shouts and screams. The family are convinced that it was carried out by the IRA. Most people are convinced it was the IRA.” The attack had been to avenge two “minor, youthful” incidents that Mr Quinn had become involved in with the son of a leading republican and an associate of high-ranking republicans, Mr McAllister said.

Rumours that the beating was linked to a multimillion-pound diesel smuggling operation over the Irish border were nonsense, he said, adding: “All he did was drive a truck. That [the smuggling allegation] is just a smokescreen, a red herring. I’m inclined to think that it was a punishment beating gone wrong.”

Mr Adams, however, said that Mr Quinn had been involved in illegal diesel smuggling. “I don’t care who was involved, they should be subjected to due process. This was a premeditated action. It clearly had to do with other criminal activity with fuel laundering or smuggling. It appears to me . . . that this young man got caught up in that and as a result he was cruelly murdered. It will not emerge that the IRA was involved. I am absolutely sure that not only is there no republican involvement in this but there is obviously no IRA involvement.”

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