A Co Clare man who killed his 17-year-old sister with a hammer in 2004 will be released from Dublin's Wheatfield prison at the end of this month, the Sunday Tribune has learned.

Patrick 'Pa' O'Dwyer, then 19, of Shrohill, Ennistymon, Co Clare, was found not guilty of murder but guilty of manslaughter by reason of diminished responsibility by a jury in 2007.

He had pleaded not guilty to murdering his 17-year-old sister Marguerite at the family home on 29 November 2004. He was the first person accused of murder to have proven their responsibility for the killing was substantially diminished due to a mental disorder. He was sentenced to six years in prison.

O'Dwyer is described as a model prisoner. "He's engaged with the psychiatric services and aside from that, he just gets on with serving his sentence. We never had any problems with him. He has a lot of visits from family and friends," said a prison source.

Defence psychiatrists gave evidence that the accused suffered from a mental disorder called depersonalisation disorder. Its symptoms include detachment or estrangement from oneself, sensory anaesthesia, lack of effective response and lack of emotional response.

On the evening of the killing, O'Dwyer, depressed and embarrassed about his drunken behaviour at a party in his house the previous Saturday night, planned to "bash his brains out" with a hammer but feared Marguerite would prevent him from doing so and was taken over by a feeling of being in a "movie".

As she watched television, O'Dwyer hit Marguerite six times with the hammer. He then went into the kitchen, picked up a scissors and a knife and stabbed his sister 90 times After going for a walk, he went to his bedroom and wrote the words 'Butcher Boy' on the wall with blood he drew from his arm with a scalpel and tried to commit suicide.

Paddy and Clare O'Dwyer have stood by their son throughout his imprisonment. Speaking on RTÉ radio last September, his mother said: "The fact that he is on medication and has responded so well I would be comfortable to have him home. I am very relieved that this is under control and will be. He will have ongoing medical care for the rest of his life. All you have is today. Yesterday is gone and tomorrow may never come."