Garda Commissioner Nóirín O'Sullivan said she is retiring after an "unending cycle" of scrutiny amid efforts to rectify the failures and mistakes of the past.

Ms O'Sullivan issued a statement earlier today announcing that she is stepping down with immediate effect.

The move comes after several garda controversies, including the falsification of garda breath tests, financial irregularities at the Garda College and the Maurice McCabe affair.

Taoiseach Leo Varadkar said Ms O'Sullivan "has given many years of dedicated service to the State".

"She has overseen many significant developments in often challenging circumstances, and in recent years took on the challenge of reforming the gardaí."

Mr Varadkar said that Ms O'Sullivan said her decision to retire was made in the best interests of An Garda Síochána. However, the commissioner did not actually say that in her statement.

A spokesperson for the Taoiseach clarified that Mr Varadkar was reflecting the sentiment expressed in the commissioner's statement.

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"The Government will now consider how best to accelerate the crucial and essential reform programme in the months and years ahead," added Mr Varadkar.

Minister for Justice Charlie Flanagan said Ms O'Sullivan faced significant difficulties, many of which had built up over several decades.

Mr Flanagan said that he is appointing Deputy Commissioner Dónall Ó Cualáin as Acting Commissioner, with effect from midnight tonight.

"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," said Mr Flanagan.

"I will brief the Government at the next Cabinet meeting," he added. 

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"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," Ms O'Sullivan said in a statement.

"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," said Ms O'Sullivan.

Ms O'Sullivan said she had considered applying for a top job in Europol during the summer but decided instead to take some time out with her family.

"I may decide to take on some other interesting and exciting challenge down the line," she said.