John Courtney must have thought his days in the witness box were behind him. He retired 19 years ago, after a long, distinguished career in An Garda Síochána, having investigated over 100 murders.

Last Tuesday, he gave evidence in the Court of Criminal Appeal. He walked slowly yet deliberately to the witness box to excavate his role in a 40-year-old investigation into the murder of a teenage girl which ripped asunder a community in Co Meath.

Courtney was a long-serving senior member of what was known as the murder squad. Its official title was the Investigative Section of the Technical Bureau of the force. The murder squad did some trojan work in investigating serious crime. It was also at the centre of a whole series of allegations of garda brutality. Certain officers attached to the squad, along with others, came to be regarded as an entity that was described in court as the 'heavy gang'.

These were officers whose names kept appearing in allegations of assault in custody, perpetrated in order to extract confessions from suspects. Allegations were made against Courtney in various courts and a tribunal. There is no record of any court or inquiry ever accepting that Courtney abused suspects. He was never charged with assault and has always maintained that he never ill-treated anybody in custody.

"There was no such thing as the heavy gang in the Garda Síochána," he said in court last week.

The murder squad investigated practically every serious crime in the country through the 1970s and 1980s. The kidnapping of Dutch industrialist Tiede Herrema in 1975 was a success for the squad when the kidnappers were located at a house in Co Kildare. Conor Cruise O'Brien, who was a government minister at the time, later related that one officer told him a suspect had refused to give the kidnappers' location until he, the officer in question, "beat the shit" out of him.

The murder squad also investigated the Sallins train robbery, for which Nicky Kelly was convicted. There was ample evidence that at least four suspects, including Kelly, were severely assaulted in custody, but the gardaí maintained they had beaten each other up. Three men were convicted after a bizarre judicial process. Two were released on appeal, and Kelly was pardoned.

Then there was the Kerry babies, an investigation that Courtney oversaw. A young woman, Joanne Hayes, and four of her siblings, were interviewed in Tralee garda station in 1984 some weeks after a baby's body was found on a beach in Cahirciveen some 40 miles away. Hayes admitted stabbing and throwing the baby into the sea off Slea Head, across the bay from where the body was found. Her brothers and sister admitted complicity in the crime. Afterwards, blood tests showed that Hayes and her boyfriend at the time could not have conceived the Cahirciveen baby. Another baby, who had died naturally soon after birth, was buried on the Hayes' farm.

The family claimed they were intimidated in custody and Hayes' two brothers claimed they were assaulted. A murder charge against Hayes was dropped and a tribunal set up to establish how a family could sign confessions to the murder of a baby to whom Joanne couldn't have given birth. That question was never properly answered in the tribunal report.

The case heard last week bore some echoes of other controversies. Martin Conmey is one of a number of people who claim they were assaulted in a garda station in the investigation into the death of 19-year-old Una Lynskey in 1971. Lynskey disappeared after alighting from a bus around 7pm on 12 October near her home in Co Meath. Her body was found in the Dublin mountains on 10 December.

The case occurred a few years before the alleged activities of the heavy gang began to seep into the public domain. Conmey was convicted of manslaughter and served three years. His fellow accused, Dick Donnelly, was convicted but released on appeal. A third suspect, Martin Kerrigan, never got to court.

He was abducted 10 days after Una Lynskey's body was found. His body was located near where Lynksey's had been found. Her two brothers and their cousin were convicted of Kerrigan's manslaughter. They claimed that Kerrigan was still alive when they left him. Pathological evidence suggests there was an attempt to castrate him within ten minutes of his death.

All of the families were drawn from a close-knit community around the Porterstown Lane area near Fairyhouse. The three suspects always maintained that they had nothing to do with Una's disappearance. The main plank of evidence against them was admissions made by Conmey and Kerrigan at Trim garda station 12 days after Una Lynskey disappeared.

Middle-aged man in a Zodiac

The murder squad, including then detective sergeant John Courtney, was drafted in days after Una's disappearance. Witnesses had given statements claiming to have seen a middle-aged man driving a Zodiac car in the area at the time in question. There was evidence of screams having been heard. One witness said he saw two men in the car, one in the back seat attempting possibly to kiss a woman beside him. Initial attempts to trace the car were unsuccessful.

Within days, attention switched to Dick Donnelly, a local 23-year-old, who drove a Zephyr, a similar model to a Zodiac. He had been accompanied that evening by Kerrigan and Conmey, who were 20 and 19 respectively.

On 25 October, the cops down from Dublin visited one witness, Seán Reilly, at a building site where he worked. Reilly had already given a statement that he was sitting in a car outside his family home with a friend at 7pm on the day in question, minutes after the last sighting of Una Lynskey. The car was facing away from the road.

His original statement was taken by local gardaí. He said a car "would have passed" going down towards the Dublin-Navan road. He said it was dark and it would have been "impossible" to make out who was driving it.

Made to feel like a suspect

His second statement was taken after he was brought to Trim garda station by members of the murder squad four days later. He wasn't under arrest, but he didn't know that he was free to leave at any time. This dilemma was experienced by many suspects – including Joanne Hayes' family – who were brought in those years to garda stations by the murder squad.

Reilly claimed in the current hearing that he was assaulted while in the garda station. He told the court he was made to feel like a suspect. He said he was punched several times on the shoulder and shouted at and intimidated by gardaí who wanted "information about Dick Donnelly's car".

After five hours he signed another statement, that referred to the car that had passed his house that night, saying it "could have been Dick Donnelly's". His friend, Martin Madden, had given a similar earlier statement. After five hours in the garda station, he changed it, saying, "A car passed going towards the Navan Road. I said to Seán Reilly, 'that must be Dick'. I know Dick Donnelly's car well, it's a silver Zephyr".

Asked in court to explain these crucial changes in statements, Courtney replied: "It often happens that we had to go back to witnesses four or five times to get the full facts from them." He denied that anybody had been abused at the garda station.

Crucially, there was no explanation in the second statements as to why they differed so radically from the first, in which there was absolutely no reference to Donnelly. Retired detective Michael Fanning was asked by Conmey's counsel, Michael O'Higgins, about the discrepancies.

"These statements are being used as a gateway to bring men in, have them charged with murder, which carries life imprisonment, and there is no explanation given as to why such changes were made?"

The detective said he couldn't answer that.

With a positive sighting of Dick Donnelly's car tied down, the gardaí had something to work on. Within hours of Reilly and Madden making their revised statements, the three suspects were brought to Trim. Martin Conmey was visited at home at 10.30pm. Again, they were not under arrest as the gardaí at the time had no power to arrest them. All of them believed they were in custody.

Donnelly made no statement during over 40 hours in custody. Conmey and Kerrigan signed statements saying they encountered Una in the lane that night and there was an incident and Una died and they hid the body. They mentioned a local field and Lucan. It was all very vague. (Una's body had still not been found in the Dublin mountains at that time.)

Conmey was kept awake from 10.30pm on Monday until the early hours of Wednesday morning.

John Courtney was also involved in another murder investigation in 1976 in which a suspect, Christy Lynch, was interrogated for 22 hours without break. Lynch claimed he had been abused in custody. The Supreme Court overturned Lynch's murder conviction on the basis of the interview conditions.

"I'd be very concerned if somebody [being questioned] didn't get a reasonable amount of sleep," Courtney told the court last Wednesday.

Earlier, Conmey told the court he had been punched, thrown on the floor and "pulled up by the hair" while being questioned. He said he was "making up stories" because he was "so frightened and scared". He just agreed with questions being put to him at the station because he had been told he would "never get home to see his parents".

At Conmey's murder trial, the gardaí accepted that he had displayed a bald patch after his time in custody, but they maintained it was a result of Conmey pulling out his own hair in frustration. A few years later the same explanation of self-infliction would be used by gardaí to explain injuries to the Sallins suspects.

Asked last week about Conmey's missing hair, Courtney replied: "I can't remember anything about it. I used no violence whatsoever on him [Conmey]."

Brutally killed the wrong man

Martin Conmey and Dick Donnelly were found not guilty of murder but guilty of manslaughter after a 13-day trial on 15 July 1972. The verdict was returned at 2.30am, after 11 hours of deliberation.

A few months earlier, Una Lynskey's two brothers, Seán and James, were convicted of Martin Kerrigan's manslaughter. If Conmey and Donnelly had been acquitted, it would have opened the horrifying vista that the Lynskeys had brutally killed the wrong man over their sister's death. Donnelly, Conmey and Kerrigan's family have always maintained that the three had absolutely nothing to do with Una's disappearance and death.

Donnelly was released on appeal, but Conmey served three years. He is now applying for a miscarriage of justice certificate to clear his name.

After the traumatic events, the Lynskeys moved away from Porterstown Lane. Una's body was exhumed from Ratoath cemetery and brought with them, while Seán and James Lynskey were serving their sentences.

Conmey's sister Mary married Una Lynskey's first cousin, Pádraig Gaughan, who had been among the crucial witnesses. Despite family ties, the Gaughans have always believed in the innocence of the three men.

Throughout the recent hearing, many of those who were in their teens and early 20s at the time are now, in late middle age, seeking what they believe to be justice.

Conmey, Dick Donnelly, Martin Kerrigan's sisters, Eileen and Kate, and Pádraig and Mary Gaughan have attended, together with other members of their families. At the opposite side of the room, Una Lynskey's sisters and their brother Seán have also been present. The bonds sundered in October 1971 have never been mended.