The Court of Criminal Appeal has quashed the conviction of Co Mayo farmer Padraig Nally, who was jailed for six years for shooting a Traveller dead on his land in 2004.
The three-judge court ordered a retrial and granted Mr Nally bail pending the new trial.
Mr Justice Nicholas Kearns, presiding, said that the court had little doubt that had the prosecution allowed the trial to proceed in the usual manner the trial judge would have given appropriate directions to the jury in the usual form.
The usual form would have enabled the trial judge to express his opinion that the amount of force used could not in his view be objectively justified in the context of the defence of self defence but would have left the ultimate decision on that issue to the jury.
The judge said that as events transpired the jury was denied the opportunity to return a verdict of not guilty, even if such a verdict may have flown in the face of the evidence and however inappropriate the trial judge might have considered such an outcome to be.
"The question whether the amount of force used is objectively reasonable is quintessentially a matter of fact for the jury,'' Judge Kearns said.
After the three-minute hearing Nally consulted with his lawyers Mr Brendan Grehan SC and Mr Michael Bowman BL before being escorted from the court by prison officers. Supporters shook his hand and offered their support as he left the crowded court.
Nally (aged 62) was sentenced to six years in jail last November for the manslaughter of John Ward, a father of 11.
In July last year, a jury found Nally, of Funshinaugh, Cross, Co Mayo, not guilty of the murder of Mr Ward, but guilty of his manslaughter. Mr Ward (aged 42) had been shot twice and beaten with a stick.
The second and fatal shot was fired after Mr Ward, from Carrowbrowne Halting Site, on the outskirts of Galway city, had left the farmyard on October 14, 2004, and was limping down the road. During his trial, Nally told the Central Criminal Court that he never intended to kill Mr Ward.
In his defence, he told the jury that in the 18 months before the shooting there were two break-ins at his property, that he had been growing increasingly paranoid and fearful and believed his life was under threat.