A Dublin man who killed his elderly father with a World War I sword hours after being released from prison has been found not guilty of his murder by reason of insanity.

Both the prosecution and the defence had called for the special verdict, after detailed assessments of the accused by consultant psychiatrists at the Central Mental Hospital.

Edward Boylan Jnr, 31, of no fixed abode admitted causing the death of Edward Boylan Snr on 6 or 7 January 2012.

However, he pleaded not guilty to the 74-year-old's murder at his flat in the Ravensdale Close retirement complex in Crumlin.

His trial at the Central Criminal Court heard that he had been diagnosed with schizophrenia more than a decade earlier and had received treatment.

He had also been deemed a "special needs" prisoner while in Mountjoy Prison, where he was serving a sentence for a minor offence until the day before the killing.

The court heard that he was in a special wing with supervision and that prison medics were aware that he was hearing voices.

He told the doctors assessing him following his father's death that he was constantly asking for medicine during his time in Mountjoy, but that it was not given to him.

Family members noted a dramatic deterioration in his mental health while in prison and, knowing that he would be homeless, had sought help from the authorities ahead of his release.

However, on the morning before he killed his father, he was released from Mountjoy Prison "without appropriate supervision, without medication and homeless", said his barrister in her closing speech.

Caroline Biggs SC said that he was in a "manic, psychotic state" at the time, having been in a padded cell there.

The trial heard that he went to his father's flat that evening. The following morning, he stabbed the pensioner 13 times in the chest with a 100-year-old bayonet, leaving the family heirloom protruding from his body.

He left, bought lighter fluid and matches, before returning and setting fire to the flat.

Psychiatrists for both sides testified that he was suffering from a mental disorder and met the criteria for an insanity verdict when he stabbed his father to death.

Boylan had told them that he had noticed people's eyes changing colour in the weeks leading up to the killing.

He explained that if they turned black, it meant they would kill him; if they turned red, it meant they were evil; and if they turned sea blue, it meant they would torture him.

He said that he had asked his father for money that morning, his father became annoyed and they argued.

Boylan said that he was about to leave when his father got out of bed, picked up a sword and began shouting at him.

He said his father's eyes became black, that the elderly man grew in size and acquired a haircut like a Viking.

Boylan said his father thumped him in the back with the sword and that he snapped.

He has since been diagnosed with schizoaffective disorder and both experts were satisfied that he was unable to refrain from his actions due to his mental disorder.

They said he therefore fulfilled the criteria for the special verdict of "not guilty by reason of insanity".

One of the doctors also felt that Boylan would not have appreciated that what he was doing was morally wrong.

Mr Justice Paul Carney told the jury that all the evidence in the case supported a verdict of "not guilty by reason of insanity".

The seven men and five women took just 14 minutes to reach a unanimous decision in favour of that verdict

The judge committed Boylan to the Central Mental Hospital for preparation of a report on his future treatment.

That report is due in court on 19 May.