The last time Gary Douch saw his mother was a few days before his 21st birthday. He told her then she was looking well. Maggie Rafter had just got her nails done; her dark brown hair was streaked with fresh highlights. She wore new jeans and a light lemon top. She had just returned from a holiday and was feeling rested and upbeat. That afternoon they both sat together in the Mountjoy visitors' area chatting, sharing a packet of Silvermints.

Gary was dark haired like his mother, a pallor under his sallow skin. As he leaned forward in his chair, Maggie noticed the bruises and cuts on his arms – his war wounds he called them. His anxious mother asked him to get some help, anger management perhaps. He had already been stabbed twice in prison. That afternoon Gary was hyper, constantly on the move, speaking a mile a minute, even more excitable than usual. He was full of his upcoming birthday and asked his mother to organise a big, traditional 21st party.

Maggie assured him she would give him a 21st birthday party, but he would have to wait until October of that year when he would most likely get early temporary release from his sentence in Mountjoy.

Two years before he had been given a three-year prison sentence for an accumulation of offences: driving without tax and insurance, possession of drugs and breaking a barring order against his own mother. Maggie was glad that Gary was behind bars because she believed he'd be safe there. Unlike other boys who had once been his schoolfriends, Gary would live to see his 21st, but only for eight more days.

In the words of the local parish priest in Darndale who had known him since he was a child, Gary "wasn't an easy kid. He had a difficult background, he didn't like school, he would have been in and out of state institutions all his teenage years and drink would have played a big portion in his life."

Maggie knew this better than anyone. Divorced from her husband, Gary's father, she had gone back to her maiden name, Rafter. Gary's father had left when he was a little boy. Until the age of 14 he had lived in the vast estate of Darndale, a bleak, exposed housing development on Dublin's northside.

Then, his mother, along with her new partner, moved the entire family to Tallaght. Gary didn't want that. He loved Darndale, his friends there, the green where they rode their horses. There, with the boys he grew up with, he was seldom called Gary; they named him "Niga", because of his sallow skin.

A couple of days after the move to the southside, Gary closed the door of the new family home behind him and walked the M50 back to Darndale, jockeying a horse on the way. After that he lived from house to house. The neighbouring women, charmed by his smile and high spirits, would call him into their kitchens and give him his dinner. His best friend Darren remembered this time. "The way Gary was, people liked him that much they let him stay in the house. It would be like, oh yeah Niga, come in, there's a bit of grub. I took him into my place because he got on very well with my ma. My ma loved him, she didn't mind him staying. I let him stay in a spare bed. He was living with me for about three years."

During his teenage years his behaviour became increasingly erratic; he dropped out of school and began to drink heavily. There was already a barring order in place when one night, 19-year-old Gary walked out to Tallaght, jumped up and down on the car of his mother's new partner and threatened to burn their house down. She called the guards.

He was sentenced to three years in prison, 12 months of that for breaking the barring order, the rest for possession of drugs and driving without tax or insurance. By July 2006, he was into his third year of the sentence.

On the night of his death, 31 July, Mountjoy prison was bursting at the seams. A Victorian jail, built 150 years ago, each cell measured 8ft by 12ft. When it was built the thick stonewalled cells were single occupancy only, with a toilet in each. Today they have long since doubled up, bunk beds in every one. Many years ago the original Victorian plumbing was deemed inadequate and so blue plastic buckets replaced the toilets.

There were 525 men in Mountjoy prison the night Gary Douch was killed; capacity at the time was 470. That night the prison was hot and airless, tempers were frayed, the place was filthy. It was difficult for prison officers to keep an eye on it all. Weapons were fashioned ingeniously; a blade from a razor attached to a toothbrush, the corner of a breakfast tray broken into a blade. Violence was casual and cruel. Before that night Gary Douch had twice been stabbed. This wasn't unusual for Mountjoy inmates.

Darren remembers Gary at this time. They had moved from sharing a house to sharing a cell. "The two of us were like brothers in the cell, the two of us looking after each other. He'd have David Gray blaring in the morning. He'd run around the landing... he'd talk to anyone. Anyone new that came in he was straight over to them. He knew everyone and everyone knew him."

But that summer Darren had moved to the training unit in Mountjoy and he wasn't there to watch his best friend's back.

Both prisoners and prison officers were jittery that night. Gary was hot, hyper and in bad humour. The toxicology results of his post mortem found alcohol, traces of a morphine-based drug and anti-depressants. Obviously, despite being in a prison, he had managed to celebrate his 21st birthday. He had also managed to annoy somebody in the process. Drunk, high, bored and cooped up, it's not clear who he was fighting with on B Wing that afternoon, but by the evening he had asked to go down to the basement and stay the night in one of the protection cells. He couldn't have made a worse decision.

When Gary was brought down to the basement that evening the prison was so overcrowded that all the protection cells were already full. He was instead brought to holding cell number two. The list of the occupants was on a page on the wall. Gary saw a name there and tried not to go in. There were six other men to share that one cell with Gary that night; one of them was his killer, Stephen Egan.

The job of the holding cell is to simply house a man as he enters prison, is processed, given his kit and allocated a cell. This whole procedure should take under an hour. In reality, the holding cells in Mountjoy were being used as overspill accommodation.

As far back as Christmas 2003 this was setting off warning bells with the Prison Officers Association, which wrote to Governor John Lonergan telling him of its deep concerns about the place. "A particular area of concern are the two holding cells in the B base area. The all too frequent use of these cells to house anything from 10-20 inmates is shocking."

The then Inspectorate of Prisons, the late Justice Dermot Kinlen, warned in his 2004 annual report that "the holding cells were never intended as sleeping quarters for prisoners... The officers and prisoners say there is frequent violence." In the spring of 2006 another letter was written by the Prison Officers Association. "Animals would not be held in such conditions," it read.

Each time concerned parties brought up the issue of chronic overcrowding with the Department of Justice or the prison service, the answer given was Thornton Hall – the 150-acre farm bought by the government in January 2005 for the astronomical fee of €30m. Four years later, in March of this year, it emerged – and only when Brian Purcell, the head of the prison service, appeared in front of the Oireachtas Public Accounts Committee – that a further €10.5m had been spent on the project. Despite all the money spent, €40.5m in all, Thornton Hall Prison remains a notion. To glimpse the site from the outside, it has all the appearance of an abandoned farm. On Tuesday of last week the justice minister, Dermot Ahern, pulled the plug on Thornton Hall. Back to the drawing board, he said. The project will be retendered. How long this will take is anybody's guess.

By the time Gary Douch came down to the basement, Stephen Egan had been in that holding cell for three days. He was over six foot tall, broad-shouldered, and weighed 15 stone. This wasn't Egan's first time in Mountjoy. The previous December he had been removed from the prison system as his increasingly aggressive behaviour was becoming a worry to the prison services. He was transferred to the Central Mental Hospital in Dundrum. There, doctors put him on psychotic medication. By the summer of 2006 he was feeling better and keen to leave the mental hospital and go back into prison. Dundrum agreed to his request.

He spent most of July in Cloverhill prison, steady enough, taking his medication, but on 29 July, the 23-year old was moved back to the already overcrowded Mountjoy Prison.

That night, six more men joined Egan in holding cell two. Unlike the thick stone cells upstairs, in the basement there were no bunk beds. The men tried to claim some space; there were three mattresses between the seven of them. At 10 o'clock all lights went off in the hot and stuffy room. There was no natural light; the only source of comfort was a thin blue strip of light coming from high up on the wall. Prisoners who have been kept in that cell vouch for its claustrophobic feel. The filth. The food scattered on the ground, the dirty tissues, the mice. The washbasin that is used as a urinal. It is still not known what happened that night between the seven men, but on the morning of 1 August, only six walked out to breakfast.

By the time the paramedics arrived Gary Douch was in a state of cardiac arrest. He was covered in blood, his eyes swollen beyond belief. There were stab wounds all over his head, shoulders, arms, thighs. His upper body was covered in the excrement of his killer. He was rushed to the Mater hospital but died soon afterwards.

Last month Stephen Egan cut a lonely figure in court. A huge bulk in the elegant room, he sat passively as Gary Douch's mother, brothers and sisters watched on. It emerged at the trial that, for those crucial days in the holding cell, he was not given his medication. Consultant psychiatrist Professor Fahy, a prosecution witness, told the court that on 31 July Egan was in the grip of a profound mental disorder. He had paranoid persecutory beliefs involving sex offenders. By the night of 31 July these were in full flight and Gary Douch got the brunt of them.

The trial in the Central Criminal Court took only two days. Both the state and the defence agreed that Stephen Egan should face a plea of guilty of manslaughter by reason of diminished responsibility. It took the jury less than three hours to agree. Egan was led away, back to the Central Mental Hospital. Gary Douch's family didn't make a statement, refused to speak to the press, and went home.

Shortly after her son's death Maggie Rafter climbed the 30 steps up to Governor John Lonergan's office. "How come," she asked the officials gathered there, "how come you put seven men together in that tiny room?"

It's May now. The sun is shining down on the cobblestones of Dublin Castle. Through the grand arch tourists take pictures of the stone statue of Lady Justice, the scales balancing on her outstretched arm. A few metres to her right there's a large blue door. It is shut to the public. A simple white sheet of paper reads, "Commission of Investigation into the Death of Gary Douch". Perhaps behind that closed door Maggie Rafter's question will finally be answered.

Ann Marie Power is a documentary maker and producer at RTE Radio 1. Her documentary '21 years and 8 days', on the life and death of Gary Douch, can be heard at www.rte.ie/radio1/doconone/