Dariusz Alchimionek, 44, had denied the manslaughter of John Gorman, 19, and assault causing harm to his brother, Adam, on 29 December 2015.

The brothers had been returning home from Tullamore when a vehicle driven by Mr Alchimionek suddenly crossed the road into the path of their oncoming car near Ballycrystal.

Tullamore Circuit Criminal Court heard how Mr Alchimionek had become convinced that the Islamic State (IS) terrorist group was about to invade Europe and a third World War was about to begin.

Consultant psychiatrists for both the prosecution and the defence, both of whom worked at the Central Mental Hospital, agreed that Mr Alchimionek met the criteria to be found not guilty by reason of insanity.

The jury had the option of returning of three verdicts: guilty, not guilty or not guilty by reason of insanity.

The trial judge told the jury: “In light of the medical evidence, it would seem to me that you have no option but to accept … the defence of not guilty by reason of insanity is available to the accused, and in such circumstances, you are obliged to acquit.”

After two hours and 50 minutes of deliberations, the jury returned majority guilty verdicts of 11-1 on both counts, which were met with applause in the courtroom.

Mr Alchimionek was then sentenced to nine years imprisonment with the final three suspended by Judge Keenan Johnson in October 2017.

The Court of Appeal quashed the jury verdict last week on grounds it was “perverse” and against the weight of the evidence.

Mr Justice George Birmingham, president of the Court of Appeal, said the quashing of a jury’s verdict because it was perverse was “very exceptional” but the court felt “compelled to do so” in this case.

The three-judge court had the option of directing a retrial or to substitute the appropriate verdict, under section 5 of the Criminal Law (Insanity) Act 2006, as had been sought by Mr Alchimionek’s lawyers.

Mr Justice Birmingham said yesterday that the court was “very conscious” of the impact the terrible tragedy had on the family of the deceased.

However, he said it was “never in controversy” that Mr Alchimionek was in fact insane at the time of the incident and, if the court directed a retrial, it seemed “inevitable the evidence would be the same”.

If another jury was to reject the unanimous expert evidence a second time, he said the Court of Appeal would “very likely” find itself dealing with another perverse verdict.

On the other hand, if another jury in a retrial acted on foot of the unanimous expert evidence, then the verdict would be the same as the Court of Appeal could enter now.

Mr Justice Birmingham, who sat with Mr Justice John Edwards and Mr Justice Brian McGovern, said “prolonging matters towards an inevitable conclusion” by directing a retrial “would not be in the public interest”.

Mr Alchimionek was committed to the Central Mental Hospital for an assessment to be carried out. The most up to date medical report was prepared in July 2017.

The case was put back to 7 March for mention.

Counsel for Mr Alchimionek, Kenneth Fogarty SC, along with Niall Flynn BL, said a retrial was an “unnecessary exercise”.

Ruaidhrí Giblin, Ireland International News Agency Ltd.