Update 9pm: An Taoiseach, Leo Varadkar, has paid tribute to the "many years of dedicated service to the State" by retiring Garda commisisoner, Noirin O'Sullivan.
Mr Varadkar said he wanted thank her on behalf of the Government and the Irish people.
"She has overseen many significant developments in often challenging circumstances, and in recent years took on the challenge of reforming the Gardai," he said.
"As she said in her statement, her decision to retire is made in the best interests of An Garda Siochana and ensuring that it can focus on the extensive programme of reform that is now underway.
I wish Noirin every success in whatever she does in the years ahead."
The Taoiseach said the government would now consider how best to accelerate the crucial and essential Garda reform programme in the months and years ahead.
"The Cabinet will discuss how best to proceed at its regular meeting on Wednesday," he concluded.
Meanwhile, the outgoing Garda Commissioner has told of how years of controversies have "personally" impacted on her.
In a message to garda members Ms O'Sullivan said tonight she was "doing a job I love" but acknowledged there had been "low points".
"Today, after 36 years proud service, I leave An Garda Síochána. I have been privileged to meet incredible people inside and outside the organisation," she said.
"During my service I have seen radical changes in Ireland, our economy and the society in which we live and work.
"The types of threats, concerns and issues have changed radically over that time but I am proud that our organisation has changed and adapted to respond to the community. I had many moments along the way where I realised, even at the time, that I was fortunate to be doing a job I love; and like any life lived there have been some low points as well.
"The last number of years have impacted on all of us personally and on the organisation, but we used those times to identity what we needed to do for the future," she added.
She thanked her colleagues for their "courage, hard work, dedication and commitment" and wished them success in their future careers.
"Over the last three years we have worked to rebuild and restructure the organisation to meet the unprecedented policing and security challenges we face," she said.
"It hasn't been easy, we can't fix everything overnight, but, we have made significant progress, some of which we are only beginning to see.
"I want to thank you for your courage, hard work, dedication and commitment in meeting these challenges. Despite the controversies and crisis our work has seen public trust and confidence remain high. We should never take this for granted.
"Each of you has a role to play in ensuring that public support continues and grows, through your attitude, behaviours, action and professionalism, regardless of the position you hold within the organisation. Always remember you can make a difference not just in perceptions but in other people's lives.
"As I leave today, I wish all of you success in your chosen career path and ambitions and continued success in your collective efforts to keeping our communities safe," she added.
Update 6.10pm: Reacting this evening to the retirement of Garda Commissioner Nóirín O’Sullivan the Minister for Justice, Charlie Flanagan, has expressed his gratitude to Ms O’Sullivan for her public service to the State.
In a statement Mr Flanagan said that Ms O'Sullivan had distinguished herself over the course of an accomplished 36 year career in a variety of roles which ranged from under-cover detective work in Dublin’s inner city in the 1980s to being appointed to the most senior position in the service in March 2014.
Minister Flanagan said that in accordance with the provisions of section 32 of the Garda Síochána Act
2005 he would appoint Deputy Commissioner Dónall Ó Cualáin as Acting Commissioner with full powers with effect from midnight tonight.
He went on: "In the coming weeks I will consult with the chair of the Policing Authority about a process to identify and appoint a permanent Commissioner to An Garda Síochána. I will brief the Government at the next Cabinet meeting."
The Minister for Justice and Equality said that since the establishment of An Garda Síochána, the role of Commissioner has been a hugely demanding and acknowledged that, during Commissioner O’Sullivan’s tenure, she was faced with particularly significant difficulties, many of which had built up over several decades.
"Commissioner O’Sullivan showed enormous resilience, determination and integrity in addressing those challenges and, in particular, in instituting a radical reform programme to modernise our policing service with the aim of providing the people of Ireland with world-class policing.
"As Minister for Justice and Equality, I will continue to press ahead with that necessary reform programme, informed by the work of the Commission on the Future of Policing in Ireland and supported by the Policing Authority and the Garda Inspectorate.
Minister Flangan went on: "I have no doubt that the men and women of An Garda Síochána who serve Ireland in the front line of policing have the appetite to embrace and drive that change.
"I wish Commissioner O’Sullivan and her family well in her retirement," he concluded.
Earlier: Garda Commissioner Nóirín O’Sullivan has announced that she is retiring from An Garda Síochána.
In a statement tonight Ms O’Sullivan said she was retiring after 36 years of privileged, enjoyable and proud service.
Ms. O’Sullivan notified Taoiseach Leo Varadkar and Justice Minister Charlie Flanagan this afternoon, thanking them for their continued confidence in her. She also thanked former Taoiseach Enda Kenny and former Tánaiste and Minister for Justice Frances Fitzgerald.
"The support for me to continue in the role is evident,” the Commissioner said today.
"However, I devoted much of my summer break to considering if continuing would be the right thing to do. It has become clear, over the last year, 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, and dealing with inaccurate commentary surrounding all of these matters.
"They are all part of a new – and necessary – system of public accountability. But when a Commissioner is trying – as I’ve been trying – to implement the deep cultural and structural reform that is necessary to modernise and reform an organisation of 16,000 people and rectify the failures and mistakes of the past, the difficulty is that the vast majority of her time goes, not to implementing the necessary reforms and meeting the obvious policing and security challenges, but to dealing with this unending cycle.”
The Commissioner expressed the hope that her successor would be given the space and necessary supports in which to do the job, build on the platform that has been developed over the last three years, and move forward the Garda Modernisation and Renewal Programme, which will see An Garda Síochána become a beacon of 21st century policing.
She also said that she was not leaving her role to take up another job. In early summer, international colleagues, she said, had encouraged her to apply for the top job in Europol. Because it would have been a prestigious appointment for an Irish citizen, she agreed to consider it, but did not proceed with the application.
"I may decide to take on some other interesting and exciting challenge down the line,” she said, but for now her intention is to retire and take some time with her family and adapt to the new phase of her life.
Simultaneous with the press release announcing her retirement, the Commissioner placed on the Garda’s internal portal a message to all staff which pointed out that despite the controversies of the past few years, the general public still registers a high degree of confidence in An Garda Síochána.
The Commissioner thanked staff for their hard work and commitment in protecting and serving our communities during what has been an unprecedented and difficult time for policing.
The Commissioner said that despite the unprecedented challenges, controversies and criticisms of the last few years, she looks back on her 36 years in the service with enormous pride.
"Being a Guard is the best job in the world. You’re committed to the public good. You’re encountering people at the lowest points in their lives. You can make a difference. As long as you avoid cynicism, you can make a profound difference – for the better – in other people’s lives”