The relatives of four British soldiers killed in the 1982 Hyde Park bomb attack have won the first stage of their civil action against John Downey.
A High Court judge in London ruled that Downey was involved in the attack and bore a liability in civil law.
A further action for damages against him will be held at a later date.
Downey was extradited to England in 2014 to face a criminal trial, but the case collapsed when he produced a so-called letter of comfort from the British government.
Today, Mrs Justice Yip ruled that Downey was an "active participant" in the bombing and was jointly responsible with others for the attack, which left 31 people injured.
Announcing her conclusions in London, the judge said: "This was a deliberate, carefully planned attack on members of the military.
"I have found that the defendant was an active participant in the concerted plan to detonate the bomb, with the intent to kill or at least to cause serious harm to members of the Household Cavalry."
Squadron Quartermaster Corporal Roy Bright, 36, Lieutenant Dennis Daly, 23, Trooper Simon Tipper, 19, and Lance Corporal Jeffrey Young, also 19, were killed by a car bomb as they rode through the central London park to attend the changing of the guard.
Lawyers acting for Sarah-Jane Young, L/Cpl Young's daughter, in whose name the action against Downey has been brought, told a hearing in London last week that the families of those killed expect "justice" to be done.
Downey, from Co Donegal, did not play any part in the trial, but filed a written defence denying any involvement in the attack.
The car bomb left in South Carriage Drive killed the four soldiers as they paraded from their barracks to Buckingham Palace.
Two were killed instantly while L/Cpl Young and Cpl Bright died from their injuries within days.
Seven horses had to be put down and another horse, Sefton, survived terrible injuries.
Additional reporting PA