Updated 8.40am

The main opposition parties have welcomed the unexpected retirement of Garda Commissioner Nóirín O’Sullivan.

Mrs O'Sullivan notified the Taoiseach and Justice Minister of her decision yesterday afternoon.

In a statement, she suggested the core of her job had become an "unending cycle of requests, questions, instructions and public hearings" - adding this was getting in the way of her efforts to reform the force.

Deputy Commissioner Dónall Ó Cualáin has been appointed Acting Commissioner, while the Policing Authority will begin a search for Mrs O'Sullivan's permanent successor.

Newstalk's Political Editor Chris Donoghue spoke to Newstalk Breakfast about the situation, and highlighted some of the people who may be in the running to take over the role.

He told Paul Williams: "In terms of some names being shot about, you mentioned Assistant Commissioner Pat Leahy.

"In terms of people on the outside but aware of the gardaí [...] I've heard Judith Gillespie mentioned. She's on the Policing Authority after working 32 years for the PSNI.

"I've heard Mark Toland mentioned - he's a GSOC investigator with 30 years experience in the [Metropolitan Police]. I've heard former assistant commissioner Derek Byrne mentioned, who left the gardaí to go become the chief of police in the Cayman Islands."

The Justice Minister Charlie Flanagan has revealed he wasn't surprised by yesterday's announcement.

He told Breakfast: "The possibility of the retirement of the Garda Commissioner was flagged to me in recent weeks. Obviously it was treated as confidential until the Commissioner made that decision herself, and herself alone."

"Best interests of An Garda Siochana"

The Government will discuss the retirement at its Cabinet meeting this week.

Responding to the news, Taoiseach Leo Varadkar said in a statement: "As she said in her statement, [Mrs O'Sullivan's] 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 Noirín every success in whatever she does in the years ahead."

Mrs O'Sullivan had faced numerous calls to resign over the last year, in the wake of controversies such as the Garda whistleblower scandal and recent revelations over fake breath test data.

Fianna Fáil Justice Spokesperson Jim O’Callaghan thanked Mrs O'Sullivan for her years of public service, but said her departure paves the way for "a new chapter for An Garda Síochana".

Deputy O'Callaghan observed: “The process to find a new Garda Commissioner presents an opportunity for the Policing Authority and the Government to ensure there is more real and necessary reform within An Garda Siochána."

The Social Democrats co-leader Róisín Shortall says the commissioner's retirement must be the start of 'a wider clear out' at the top of the force.  

Sinn Féin's Mary Lou McDonald said she welcomed the "Commissioner’s decision to act - albeit very belatedly", while Labour leader Brendan Howlin also called for new leadership to bring about reform in the force.

Meanwhile, Minister Flanagan said he will consult with the Policing Authority to find a new commissioner.

The process may take months to complete.

In a statement, the authority said it "agrees with Ms O Sullivan that the modernisation and reform
programme in the Garda Síochána must continue" - adding that it will continue to work with Mrs O'Sullivan's successor to achieve that.