PATRICK 'Pa' O'Dwyer doesn't have nightmares when he goes to sleep at night. It is only during the day that he gets flashbacks from Monday 29 November 2004, the night he killed his younger sister, 17year-old Marguerite O'Dwyer.

Why Pa did it has baffled everyone who knows him or has since come into contact with him. The quiet, well-behaved then 19-year-old, from a well-known and respected family in Ennistymon, Co Clare, had always got on well with his little sister and the rest of his family.

Last week, the jury in the Central Criminal Court heard the run of events in Pa O'Dwyer's life that led him to hit his sister repeatedly over the head with a hammer, before stabbing her with a knife and scissors. Tomorrow, they begin deliberations and must come to one of two verdicts:

either Patrick O'Dwyer will be found guilty of murder, or of manslaughter on the grounds of diminished responsibility.

On the afternoon of Tuesday 30 November 2004, Pa O'Dwyer walked into the quiet garda station in Ennistymon and told stunned gardai that he had killed his sister and that they would find her body in the sitting room of the family home.

When investigating gardai came into the house, they found the sitting room in darkness and Marguerite's body lying face-up on the floor, with the right side of her skull smashed in and up to 90 stab wounds on her legs and torso.

There was a steak knife by her side, and blood on the walls and ceiling.

Upstairs in Pa's bedroom, there was blood on the ground, a knife on the dressing room table and written on the wall in blood were the words:

'Butcher Boy!'

In the station, Pa was anxious to talk about what had happened, but he couldn't explain why he had done what he had done. The gardai said he was cooperative but they were surprised by how "calm" and "cold" Pa seemed to be. "He seemed almost removed from the situation, " the court heard from garda reports. "It was more like he was a witness to the event."

"It was like being inside a room and watching a video that I couldn't turn off, " Pa told gardai.

He was "sorry" he had done it but he simply couldn't explain why he did it, the court heard.

'A quiet child' Patrick O'Dwyer, the middle child and only son of Paddy and Claire O'Dwyer was born in July 1985, two months premature. His father worked in a local factory, while his mother was a bank worker. They had one child already, Louise, and Marguerite was born two years after Pa.

Patrick was described as a "quiet child". His mother told the court, "Pa was never rebellious or moody. If I said no to something, he'd accept it." She added, "Pa doesn't have a bad bone in his body."

He was heavily involved with the local GAA club and, until he was 14, was a good student. Around this time, a few cracks occurred in the O'Dwyers' happy family life. Paddy's father, who lived across the road from the O'Dwyers, died and Paddy went through a phase of heavy drinking. He struck Claire once and the children were taken temporarily to an aunt's house, the court heard.

Whether this is connected or not, Pa's schoolwork went down rapidly and he lost interest in his studies. Although his parents patched up their problems and Pa got 395 points in the Leaving Cert, he got on badly when he moved to Galway to start college in September 2003.

He spent whole days lying in bed, smoking cannabis, and moved home after a few months, putting "a brave face on" about it to the family.

Although quiet, Pa had some close friends with whom he socialised regularly and from whom he got the nickname 'Psycho Pa' . . . so-called because of his behaviour when drunk.

In 2004, this drinking got out of hand when, in January, Patrick accosted a women he didn't know in the street and asked her to kiss him.

When she refused, he punched her in the face and bit her fingers. The woman didn't press charges, but a false rumour went around the town that Patrick had raped her.

In July of that year, at a family wedding, Pa became drunk and had a physical row with his father. After both incidents, he tried to commit suicide by tightening a belt around his neck, but no one ever found out.

According to defence witness Dr Cleo Van Velsen, consultant psychiatrist in forensic psychotherapy, there was "an impending crisis".

Inexplicable events On Saturday 27 November, Claire and Paddy O'Dwyer left Ennistymon for a short holiday in Spain. Left at home were 17-year-old Marguerite, a "bright student" who was in her final year of school, and Pa, now a butcher's apprentice.

Their eldest child, Louise, was in college in Galway.

That night, Marguerite and a few of her friends had a party in the house and Pa joined them for some time, drinking what the court heard was "a cocktail of everything". This included filling a pint glass with vodka, topping it up with half a can of Red Bull and "gulping" it down. "He could have died from alcohol poisoning, " said Dr. Van Velsen.

He ended up getting sick in his friend's car and banging his head on the sidewalk. His friends were worried and considered getting him medical attention that night before putting him to bed.

Sunday was spent "mooching around" with his sister. His mates called around and he promised one of them that he would stay off the drink until Christmas. That night, he went to bed at 10 o'clock and woke up the next morning feeling "tired and depressed".

His employer, Bernard Rogan, reported that Pa was "his usual self" that day. He arrived promptly at 9am and before he left, asked Bernard if he wanted him in at 8.30am the follow morning. Bernard said that would be great and thanked him.

It was at 11.30pm that night, when he and Marguerite had been watching TV together all evening, that Pa suddenly decided to kill himself.

In the middle of watching The Office, he told gardai, he got up and went to the kitchen where he found a hammer.

He was about to go to the bathroom with the purpose of "bashing his brains out" when he caught sight of Marguerite in the sitting-room.

He told gardai that he "thought she might try to stop me" and he went in to the sitting room.

Marguerite was lying sideways on the couch and she looked up at him and smiled. He hit her over the head with the hammer and then paused before striking her a few more times.

Marguerite didn't cry out but started making a "horrible gurgling sound, " Pa told gardai, according to evidence given in court, so he went to the bathroom and the kitchen where he got a scissors and a knife respectively. He then stabbed her repeatedly, "possibly" to put her out of her distress, before leaving the house to have a cigarette. During the entire incident, he said he didn't utter a word.

Later that night, he went to his bedroom and hit himself over the head with the hammer a few times, before cutting his wrists and writing on the wall. He fell asleep, but said when he woke up the next morning, "It was like I was still the same."

He considered setting the house on fire, but then got into the bath with the purpose of drowning himself. When this didn't work, he left the house with the idea of hitching a lift to his sister in Galway. Instead, he walked into the garda station.

Suicidal urge "He was overwhelmed by a suicidal urge and this is a murderous state of mind, even though directed on one's self, " said Van Velsen. "Very tragically for Marguerite, she was there. He had to countermanage these feelings and he exploded with them and put them on to her."

Van Velsen, who first interviewed Patrick in November 2005, told the court that she initially struggled to formulate his state of mind. "As the year went on, he would come to mind, " she told the court. "He really puzzled me."

Pa's lack of emotion, or "emotional autism" as Van Velsen referred to it, his detachment and feelings of living in a dream or movie led her, amongst other things, to make a diagnosis of depersonalisation disorder.

This, she said, was backed up by Dr Joan Moroney, consultant neurologist, who told the court earlier in the week that she believed Patrick suffered from a form of epilepsy. Epilepsy can be connected to depersonalisation disorder.

However, director of the Central Mental Hospital Dr Harry Kennedy told the court he couldn't find any evidence of epilepsy, depersonalisation disorder, or any mental disorder or illness in Patrick and that to do so was "clutching at straws". What happened was a result of "profound shame and embarrassment, exacerbated by alcohol", he said.

There was one thing that all the psychiatrists were agreed on: this was a misdirected suicide with very tragic consequences for the family involved.

Sitting side by side Throughout the entire proceedings, the four remaining members of the O'Dwyer family sat side by side. Sometimes Claire would pat Pa's knee. Other times his father would sit solidly beside him.

Mostly they looked down at their laps or put their hands over their face as the dreadful sequence of events of that night was relayed matter-of-factly. They passed hankies to each other but never spoke during the proceedings.

When the short history of family violence was dwelt upon, Claire, small and fragile, left the court. Pa's head never lifted for the entire time and he never looked at the jury.

It was Louise, the eldest child, who kept her head up more than the others. When she first saw Pa after finding out he had killed their sister, she begged him not to kill himself. "You're the only sibling I have left now, " she reportedly told him.

But Patrick has promised his family he won't attempt to harm himself again. As he told Dr Harry Kennedy: "I've put them through enough."