Dean Lyons, who died on Thursday in Manchester from a suspected drugs overdose, was living the life of a homeless heroin addict in Dublin's north inner city in July 1997 when he was arrested for questioning about the murders of Sylvia Shields (59) and Mary Callinan (61).

The two women, patients at Grangegorman psychiatric hospital, had been murdered brutally, stabbed and mutilated at their home in the hospital grounds four months earlier on March 6th.

Mr Lyons was taken to Bridewell Garda station in Dublin where he was held in cells until mid-afternoon on July 25th, 1997, when he was placed in a video and tape-recording suite where two detective gardai began questioning him at 2.27 p.m. Mr Lyons had been without heroin since the previous day.

The transcript of this taped interview shows him to be confused and incoherent. He suffered from learning difficulties at school, feared authority figures and had a habit of confessing to things he had not done. In the interview, he readily admits to every charge put to him. Mr Lyons's mother and father, who were allowed to visit him in the Bridewell station, said he appeared completely disorientated and was swaying and slurring his words when they met him. After they left he was questioned again.

As a result he made another, written, statement. It was this on which the prosecution relied when Mr Lyons was charged with the murders of Sylvia Shields and Mary Callinan. But when this statement was made, there was no video and audiotaping.

This statement contains a chronologically correct narrative about the murders. There are also accurate descriptions of the interior of the house and the actions of the murderer inside.

In this second purported confession, Mr Lyons allegedly said: "I got into the side of the house, in over a wall. I tried to break the window by whacking it with my elbow. It only cracked the first time. I then hit it again and it broke . . . Nobody came so I pulled out the glass from the window to make it easier to get in. I threw some of the glass away. But I stacked more of it in a pile so I wouldn't be heard."

This description is very close to what actually happened. The real murderer climbed over a wall at the side of the house and broke a pane in a sash window into the dining room. The murderer threw some of the glass shards into the garden and placed other pieces in a small pile on the windowsill.

The statement also contains the following: "I came up the stairs and was facing a room. I bypassed this room. I passed the next room until I got to the last room on the right. The door of this room was open. I went into the room and found a woman asleep on the bed."

The layout of the staircase and bedrooms of the house as described in these lines corresponded with forensic evidence about the crime scene.

Mr Lyons's statement was able to describe one of the victims, "wearing a light night dress, greycoloured shoulder-length hair and she was a stout woman". Again, this is a very accurate description. Both victims died as a result of frenzied, powerful stabbing. The statement is fully in line with this. It continues: "I then attacked her [the second victim] with the knife. I lost the head with her, too. I cut her up with the knife. I kept stabbing her after she stopped screaming."

There is another remarkably accurate description of this victim. "I would describe her as tallish. She wasn't as heavy as the first woman, oldish and she wore a bra and panties."

The real murderer emptied the kitchen drawers and took all the long knives and a carving fork to mutilate his victims. The Dean Lyons statement reads: "I took four knifes [sic] out of the drawer. I can describe them as a couple of electric carving knifes. The other was a black handle steak knife. I also took out a big fork, the type you stick into meat when you want to cut it."

This information was not published in the media at the time of the murders.

Mr Lyons's purported written statement is in clear, mostly grammatically correct English. It is strikingly unlike the Dublin street language in the first videotaped interview. Mr Lyons left school with partial reading and writing skills and had a very limited vocabulary.

On the basis of this "confession", he was charged with the murders. On August 15th, 1997, a Friday night three weeks after Dean Lyons's supposed confession of July 25th, a young married couple, Catherine and Carl Doyle, were brutally stabbed to death at their home in Roscommon. An English-born man, Mark Nash (24), had been visiting the Doyles with his girlfriend, Catherine Doyle's sister. For the previous two years he had been living in north Dublin.

Immediately after the Roscommon murders, Nash took off across the countryside towards Galway. He was caught by gardai within 24 hours. Arrested and inside a Garda car, he began admitting the Grangegorman murders. At first the gardai had no idea what he was referring to.

When interviewed later in Galway Garda station, Nash said he had found himself outside the house in Grangegorman the previous March 6th. "I cannot explain my mind at the time," he said. "Everything seemed to turn black. I lost control over myself and decided to break into this house."

He said he remembered breaking the "bottom right-hand pane" in the kitchen window and climbing on to a worktop. He even remembers the swing-top bin on the floor near the window.

He went into the dining room and living room which, he recalled had a "modern black-coloured Osaki television. It was an 18 inch screen".

He then said that while he was in the kitchen he picked up a "red-handled bread knife with a serrated edge. The blade was about four or five inches long". He sketched the kitchen knife accurately and also the pattern on the sole of the boots he was wearing. The boot print matched evidence gardai had from the scene in Grangegorman.

He said he turned left and went into a bedroom where there was a woman asleep in a "single or three-quarter bed".

"I pulled down the duvet off this lady, down to her waist. I think she had a cream nightdress. This lady I would describe as about mid-50s, six foot tall, very heavy build, dark hair." He says he remembered stabbing her through her nightdress.

He said he left the room and turned right down the landing and opened another bedroom door. There was a bed to his left.

"When I looked towards the bed I saw a lady getting out of the bed. She walked to the foot of the bed. I stabbed her in the chest while she was standing. I don't know how many times. She fell forward on to the bed. I may have cut her throat, I cannot remember. I will describe this lady: late 50s, grey hair, five foot and a little bit, slim build. She was also wearing a nightdress. I am not sure of the colour, it was either cream or pink."

He says he remembered leaving the light on this room. He then went into another bedroom where a third woman was asleep.

"She had black earphones on her head which I presume was for her personal cassette. I think she was wearing some type of headgear. She was fully covered by the bedclothes. This was quite a sizeable room. I still had the knife in my hand. I regained my self-control and left the room."

This was a crucial piece of evidence. It had never been reported in any newspaper, TV or radio broadcast and was known only to detectives at the time that the woman who escaped had been asleep with headphones on.

It was known that the killer had stood over this woman's bed as blood had dripped from a knife or his clothing on to the carpet beside her bed.

On October 12th, 1998, Nash was convicted of the Roscommon killings and sentenced to life imprisonment. Despite his apparent confession to the Grangegorman murders, he has not been charged with them.

Dean Lyons, meanwhile, remained in prison charged with the two Grangegorman murders. He stayed there for eight months before, on April 29th, 1998, the State Solicitor withdrew the charges, giving no reason.

The Lyons family has never received an explanation for what happened. Dean never sought compensation. Last year after serving a prison and treatment sentence for his part in a syringe robbery in Dublin in 1997, he left Ireland and went to a rehabilitation hostel in Rochdale, Lancashire.

Earlier this year, however, it is understood he began using heroin again. He died in Manchester on Thursday.

An internal Garda inquiry was held into the Grangegorman affair. The report's findings have been kept secret.