She joined the force in 1981 and her work spanned both operational and administrative areas. Much of her early years involved undercover work investigating major drugs gangs.

She was promoted to superintendent in 2000 and served in the Garda College with responsibility for specialist training.

She also worked as a detective superintendent in the Garda National Drugs Unit.

In 2003, she was promoted to chief superintendent and served as the head of the Garda Technical Bureau, which comprises experts in photography, ballistics, fingerprints and mapping.

Four years later she was promoted to assistant commissioner and served in the western region. She also worked in human resource management.

In 2009, she was appointed assistant commissioner for crime and security.

Two years later she was promoted to deputy commissioner of operations. When, on March 25th 2014, Garda commissioner Martin Callinan retired, she was made interim commissioner.

On the day of her appointment as commissioner the following November, she told the media at Garda Headquarters that An Garda Síochána had been through an “unprecedented time” following a string of controversies and a number of critical reports into malpractice and abuse of process.

She pledged to use a highly critical Garda Inspectorate report as a “roadmap” for the future of the force.

She said rank-and-file gardaí had been “hurting” and that part of the task before her would be to lift morale.

“There are certainly things that we have to do differently. There are lessons,” she said. “I don’t underestimate the big body of work that we have to do in the coming weeks and months and years.”

In a rare public interview at the Kennedy Summer School in Wexford last year, Ms O’Sullivan said gardaí had a “duty” to speak out if they saw wrongdoing in the organisation.

Asked if she encountered internal resistance after Mr Callinan’s departure, she said: “People deal with things like that in very different ways. A lot of people were shocked. A lot of people were disappointed. Initially, a lot of people were fearful. For me the job was to steady the ship – to let people vent, to let people grieve. And then to get on with it.”

With a general sense that the job of commissioner would go to “anyone but an insider”, she said she had considered not applying for the post when it was advertised internationally. She acknowledged it was challenging to lead the force when her colleagues knew there was a possibility that she would not be in the post for long.

Ms O’Sullivan is a graduate of the FBI National Executive Institute’s law-enforcement course for police chiefs. She also holds first-class honours in diploma and MA courses in business and advanced management from the Michael Smurfit School of Business at UCD.