Retired chief state pathologist Prof Marie Cassidy tells a story in her new memoir of two “hard men” driving to commit a crime of some sort. One of them is armed with a shotgun and sits with the butt of the weapon resting on the floor of the car and the barrel pointing at his head. You can guess what happens next: the car hits a pothole and the gun goes off. “Hapless becomes headless,” Cassidy tells us.

It is these morbid vignettes, rather than the accounts of the many high-profile murder cases she has investigated, that make Beyond the Tape such a compelling read.

Cassidy served as a forensic pathologist in Scotland until 1998, when she took up a post with the State Pathologist’s Office in Dublin. In 2004, she succeeded Prof John Harbison as State Pathologist, placing her at the centre of many of the most high-profile murders in the State until her retirement in 2018.

The unusually public profile of the State Pathologist in Ireland (in Scotland they operate very much behind the scenes) means Cassidy has appeared on the evening news more than most politicians. She can be recognised, she tells us, by her 3-inch heels combined with white overalls, which are typically several sizes too big.

Beyond the Tape is a pacey retelling of her work in Glasgow and Dublin as well as a primer on the history, role and methods of death investigations. (Did you know until 1962 Irish law mandated that a dead body be brought to the nearest pub for storage until the inquest – there is a reason many Irish pubs used to double as undertakers.)

Most cases the State Pathologists Office deals with are straightforward. It doesn’t take a medical degree to determine the cause of death when the victim is riddled with bullet holes. The challenge in these cases, according to Cassidy, comes in determining the events immediately preceding the event.

On Christmas Day 2008, the country woke up to the news that a woman and her two children had died in a house fire in Kilkenny. It was a terrible tragedy but not suspicious, gardaí told Cassidy. The Christmas lights had probably been left on. The next day an apologetic garda phoned her back: a preliminary examination of the bodies had raised some concerns; her services would be required after all.

The children, Nadia (2) and Zara (7), had indeed died of smoke inhalation as evidenced by the black soot in their airways. But there was no such soot in the airways of their mother, Sharon Whelan, indicating she had died before the fire.

Cassidy examined the mother’s remains and found sure evidence of strangulation. Sharon had been raped and then choked to death before the killer started a fire to hide his crimes. A tragic accident suddenly became a triple murder investigation.

A year later, postman Brian Hennessy received three life sentences for the murders. He told gardaí he killed Sharon to stop his girlfriend finding out they had slept together.

In 2006, Cassidy helped solve an even more complex case, one with all the hallmarks of an Agatha Christie locked-room mystery. Mother-of-one Siobhan Kearney was found on the floor of her bedroom with a vacuum cleaner cord fashioned into a noose beside her.

A sad but obvious suicide, it was assumed. This was supported by the ligature marks on the victim’s neck and the fact that the door of the room had been locked. The key was found on the floor near the body; Siobhan has probably locked herself in so her young child would not be the one to find her body. But the senior investigating officer had his doubts. “Something doesn’t feel right,” he told Cassidy.

Detailed examinations of the body and the scene raised concerns that it was a murder made to look like a suicide. The position of the noose and the injuries were not consistent with suicide and it looked like the key may have been slid under the door after it was locked from the outside.

Siobhan’s husband, Brian Kearney, was eventually brought to trial for her murder. During the trial the prosecution performed demonstrations for the jury to show that Siobhan could not have used the flex to kill herself. This combined with the pathology evidence led to his conviction.

Some cases are more straightforward. Cassidy describes being called to the scene of a man found dead in his hallway with what appeared to be blood on the walls and floor around him. Cassidy arrived, took one look and left again. The “blood” was red paint; a postmortem would later show the man had died from an heart attack after getting in an argument with his son over the decorating.

Remarkably, the son was charged with manslaughter as the police determined their argument had caused the death. Cassidy tells us the heart attack could just have easily happened while the victim was running for a bus. A jury agreed and the son was acquitted.

Cassidy’s memoir reads like an anthology of murder trials that have grabbed the country’s attention in the last two decades. There is the death of 11-year-old Robert Holohan in Cork in 2005, leading to the conviction of his neighbour Wayne O’Donohue for the boy’s manslaughter. There is the murder of Tom O’Gorman, who was killed by his tenant Saverio Bellante in 2014. Bellante, who was later found not guilty by reason of insanity, stabbed the victim to death before eating some of his body parts.

And, of course, there is the murder of Rachel O’Reilly by her husband Joe. Cassidy describes the gruesome murder scene; Rachel had been bludgeoned to death and the scene made to look like a burglary gone wrong. It was the savage injuries inflicted on the victim that made the pathologist suspicious. When “overkill” is present in a death, particularly in the death of a woman, it suggests a personal motive. “It smacks of rage specifically directed towards that person.”

Cassidy’s suspicions were heightened further when she saw Joe O’Reilly on the Late Late Show appealing for information on his wife’s murder. Such public displays of grief, she says, are a big red flag.

These accounts contain little new information about the murders, instead focusing on the particular challenges they posed from a forensic pathology point of view. Unsurprisingly, it can also be a difficult book to read. There is a woman who was found with her own spleen stuffed in her mouth and detailed descriptions of the decomposition process. These are typically included not for shock value but because they help explain an aspect of death investigation.

CSI addicts and avid readers of court reports will find little to surprise them in Beyond the Tape but will nonetheless appreciate the insight into Marie Cassidy’s macabre but vital craft. But perhaps don’t read it over lunch.

Conor Gallagher is Crime Correspondent of The Irish Times