O'Sullivan had been in charge of the Ireland team for six years
Eddie O'Sullivan has resigned as coach of Ireland after a disastrous World Cup and disappointing Six Nations campaign.
The 49-year-old led the Irish to three Triple Crowns during his six-year reign but his critics felt he did not get the best out of a richly-talented team.
O'Sullivan said: "Having given my role as team head coach much consideration, I have decided to step down."
IRFU president Philip Browne said he "respected the decision of Eddie O'Sullivan to resign as Head Coach".
"In accepting his wish to step down, I want to emphasise the high regard the Union has for him and the achievements he attained during his reign in office.
"His record as Ireland's most successful coach is his proud legacy and all of us in Irish rugby are grateful to him for the many memorable moments and the achievements he attained during his reign in office.
"His record as Ireland's most successful coach is his proud legacy and all of us in Irish rugby are grateful to him for the many memorable moments and achievements our senior Irish players and squads attained during his coaching tenure.
"I want to congratulate and thank him for that contribution to Irish rugby and to wish him every success in his future career."
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Highlights: England 33-10 Ireland
Declan Kidney is among those tipped to take over, but Mike Ruddock, Pat Howard, Wayne Smith, Jake White and Alan Gaffney are also in the frame.
O'Sullivan's fate was sealed by Ireland's dismal fourth place finish in the Six Nations, their worst championship performance since 1999.
They could only beat Scotland and Italy, losing 33-10 to England at Twickenham in their final game last weekend.
At the World Cup, Ireland failed to make it out of the group stages following defeats by France and Argentina.
O'Sullivan's statement continued: "I would like to thank my management team and all the players that I have worked with during my tenure as Irish coach.
"Their commitment and professionalism in representing their country has been consummate throughout.
"I would also like to thank the IRFU for their unwavering support of me as coach to the team.
"In addition, I would like to thank the Irish rugby supporters who have played a huge role in driving the team on to some outstanding performances in recent years.
"Finally, I would like to wish the Irish rugby team and the IRFU every success in the future, in a professional sport that is becoming more demanding and competitive by the day.
Kidney is the early favourite
"I will not be making any further comments on this issue for the foreseeable future, and I would ask that the privacy of both myself and my family at this time be respected."
Under O'Sullivan, who took over in November 2001, Ireland won three Triple Crowns in four years and narrowly missing out on Grand Slams in 2006 and 2007.
They also rose to number three in the world ranking in late 2006 and early 2007.
O'Sullivan's position looked secure, prompting the Irish Rugby Football Union to award him a new four-year contract before the World Cup.
The contract even contained a clause allowing him to take time off from the job if he was appointed to coach the British and Irish Lions in 2009.
O'Sullivan had initially insisted he had no intention of quitting.
After the loss to England, he said: "I believe I am the right man to lead Irish rugby.
"I think the question for me is: Have I got the hunger for this job? And the answer to that is absolutely, 100%."
Kidney is the early favourite to succeed O'Sullivan after enjoying success with Munster, steering them to the 2006 Heineken Cup crown.
The Irishman, who also coached Leinster, has already had a taste of international rugby, working for a short time as an assistant to O'Sullivan.
World Cup-winning coach White has also been linked with England
But Kidney will face overseas opposition from the likes of White, who masterminded South Africa's World Cup triumph last autumn and has been linked with England.
Former Wales Grand Slam-winning coach Mike Ruddock is another who is in the frame.
Australian Gaffney is due to step down as Saracens director of rugby at the end of the season and will join Leinster as backs coach in August, but he would be the perfect choice if a stop-gap was required.
Howard, another Australian, steered Leicester to a Premiership and EDF Energy Cup double in 2007 before returning home to manage his family's pharmaceutical company.
He was soon tempted back into the sport, becoming general manager of the Australian Rugby Union's high performance unit, though he is now quit that role.
Smith has an impressive club record, lifting Northampton at of the doldrums and guiding the Canterbury Crusaders to Super 12 glory in two successive years.
But he has endured a chequered Test career.
Appointed New Zealand coach in 1999, he was deposed in 2001 following a poor Tri-Nations.
He linked up with Graham Henry as backs coach for his second stint with the All Blacks but that ended in failure with a quarter-final exit at last autumn's World Cup.