A man who killed his mother in a knife attack has had his conviction for murder overturned by the Supreme Court.

Celyn Eadon was aged 19 in 2011 when he stabbed his 46-year-old mother Noreen Kelly, from Islandeady, Castlebar, Co Mayo, 19 times.

A five-judge Supreme Court unanimously ruled his murder conviction was unsafe.

The court found the trial judge's instruction to the jury on intoxication and specific intention was inadequate. It also wrongly instructed them that intoxication is never a defence in law.

The court found the trial judge did not inform the jury of the "vitally important point" that intoxication can be a partial defence to murder although not to manslaughter.

The Director of Public Prosecution will now decide whether to accept Celyn Eadon's manslaughter plea or pursue a retrial for murder.

Rejecting arguments that speeches from counsel to the jury, which stated intoxication could be a defence to murder, could save the defects in the trial judge's charge, he said speeches from counsel cannot supplant the trial judge's obligation to correctly state the law.

After the judgments, Patrick Gageby SC, for Mr Eadon, confirmed to the court his client had pleaded guilty to manslaughter at the outset of his trial.

The DPP must now consider whether to accept that plea, with the effect Mr Eadon will be sentenced for manslaughter, or retry the murder charge.

Mr Eadon was a significant and persistent abuser of alcohol and drugs, including amphetamines, cocaine and crystal meth, from the age of 13.

In the 18 months before the killing, he was spending upwards of €400 per week on drugs and drank large quantities of alcohol daily.

His mother had taken drugs from her son's bedroom and burnt them on the evening before she died.

Mr Eadon killed his mother in the knife attack on 9 March 2011 at about 1.30am. He had never previously exhibited violence towards her, the judgment noted.

After his plea to manslaughter was not accepted, he ran two defences at trial. One was based on diminished responsibility, which the jury rejected, and the second, related to intoxication.

He was convicted of murder and lost an appeal over the conviction.

The Supreme Court agreed to hear a further appeal.

It noted, in the week before his mother's killing, Mr Eadon was suffering from hallucinations and paranoid delusions and "behaving most strangely".

His father, from whom his mother was estranged, called to the house the evening before the killing and found his son barricaded inside and in total darkness, in the belief aliens were coming, third parties were spying on him and attempting to poison or gas him, and a nuclear explosion had taken place. 

Having killed his mother, he turned up at a neighbour's house about 7.20am, barefoot and in a wet tracksuit bottom with scratches, bruises and cuts on his body indicating he had gone there through fields.

The neighbour said Mr Eadon told him he had been abducted by aliens, the army were looking for him and he thought he had killed his mother.

In those circumstances, the defence relied upon the defence of intoxication.

A psychiatrist gave evidence he considered Mr Eadon was grossly intoxicated at the time, experiencing psychotic symptoms as a result and this would have affected his capacity to form specific intent.