Ms O’Sullivan retired from the force without serving out a notice period after a near three-year commissionership dogged by controversies.
Her announcement came as a surprise even to those she worked closely with. Her term as the first female commissioner came to an end at midnight.
Deputy commissioner Dónall Ó Cualáin, who is in charge of governance and strategy, has taken over as acting commissioner.
In a statement, Ms O’Sullivan said she believed there was support for her to continue in her role but it had become clear to her “that the core of my job is now about responding to an unending cycle of requests, questions, instructions and public hearings involving various agencies including the Public Accounts Committee, the Justice and Equality Committee, the Policing Authority, and various other inquiries” rather than implementing necessary reforms “and meeting the obvious policing and security challenges”.
Allegations that she smeared whistleblower Sgt Maurice McCabe are being investigated by the Charleton tribunal.
But it was more recent controversies concerning inflated Garda breath test data, the wrongful conviction of motorists and especially her handling of the financial governance problems at the Garda College, Templemore, that damaged her most.
Ms O’Sullivan took over as acting commissioner in March 2014 after Martin Callinan also stepped down with no notice and amid controversy. She was appointed to the role proper six months later. In total she spent 36 years in the Garda.
Taoiseach Leo Varadkar thanked the outgoing commissioner for “her many years of dedicated service to the State”.
He said he agreed that her decision was made in the best interests of the Garda, “ensuring that it can focus on the extensive programme of reform that is now under way.”
However, he said that the Government would now seek to “accelerate the crucial and essential reform programme” in the force.
Just last week, he said the commissioner retained the Government’s confidence.
Minister for Justice Charlie Flanagan said he would discuss with his Cabinet colleagues the next steps for the Garda, including the process for selecting a new commissioner.
Labour leader Brendan Howlin said it was now time for an outsider to be appointed to lead the Garda. Ms O’Sullivan was unable to adequately reform the force as she was a product of its culture, he said, and the next commissioner should come “from outside the current ranks”.
The recently established Commission on the Future of Policing in Ireland will now likely be asked to examine breaking up the force into two agencies, one looking after national security, the other traditional policing duties such as tackling crime, road traffic enforcement and other such duties.
Such a move would mean foreign candidates could be hired for senior Garda posts without the need to open State security practices and secrets to foreign nationals. At present there are only two ways into the force; by joining as a recruit or from the PSNI.
Government sources believe the remuneration package for the post needs to be increased from about €180,000 a year plus allowances, and that State security should be taken out of the Garda organisation so recruitment could be opened internationally across the many ranks of the force.
The Policing Authority, which became operational after Ms O’Sullivan secured the commissioner’s post, will now effectively recruit her successor.
Ms O’Sullivan will be entitled to a gratuity payment of close to €300,000 and an annual pension of almost €100,000.