Republic of Ireland manager Martin O'Neill and his assistant Roy Keane have left their roles.
The pair were appointed in 2013 and guided the Republic to the last 16 of the 2016 European Championship.
However, they did not win a competitive game in 2018 and were relegated from Uefa Nations League Group B4.
"I thank Martin, Roy, and the management team for the impact they had," said Football Association of Ireland (FAI) chief John Delaney.
"There have been many highlights during Martin's reign - none more so than Euro 2016 in France, which will live long in the memory of all Irish supporters."
O'Neill, who leaves alongside his backroom team of Keane, Steve Guppy and goalkeeping coach Seamus McDonagh, signed a contract extension in January to remain as manager until 2020.
The FAI statement said the governing body's board would "meet promptly to discuss the process of recruiting a new manager".
In a later statement, O'Neill thanked the "FAI board for giving me that opportunity" to manage the Republic.
"It is with a heavy heart that I leave this role. It was one of my lifetime ambitions to take charge of the Irish senior squad."
Thanking his backroom team, O'Neill described Keane as "an inspirational figure in the world of football".
O'Neill pays price for difficult 2018
After their World Cup 2018 qualification hopes ended in a humiliating 5-1 home defeat by Denmark, the Republic's inaugural Nations League campaign was plagued by poor results and a string of off-field incidents.
In September, midfielder Harry Arter pulled out of the squad for games against Wales and Poland following a row with Keane.
O'Neill defended his assistant after audio emerged of defender Stephen Ward apparently describing the incident which led to Arter's withdrawal.
Although Arter would return to the squad for October's fixtures, highly rated West Ham teenager Declan Rice has remained absent since September.
The London-born defender continues to consider whether to represent the Republic, for whom he has featured in three friendlies, or England.
Relegation to the third tier of Uefa's new competition came as they collected just two points, scoring only once, in four games.
Speaking after Monday's scoreless draw in Denmark, O'Neill indicated that he wanted to remain in the job.
The 0-0 draw in Aarhus means the Republic, ranked 33rd in the world, have gone four matches without scoring a goal and failed to win a competitive match in 2018.
|Republic of Ireland results in 2018|
|L||1-0 v Turkey||Friendly|
|L||2-0 v France||Friendly|
|W||2-1 v United States||Friendly|
|L||4-1 v Wales||Nations League|
|D||1-1 v Poland||Friendly|
|D||0-0 v Denmark||Nations League|
|L||1-0 v Wales||Nations League|
|D||0-0 v Northern Ireland||Friendly|
|D||0-0 v Denmark||Nations League|
Euro 2016 the high point of O'Neill era
The appointment of O'Neill and former captain Keane in November 2013 was greeted by a wave of optimism among supporters following the decline of the team under Giovanni Trapattoni, which had seen the Republic slump to 67th place in the Fifa world rankings.
The new management duo had almost a year to prepare for their first competitive fixture and successfully steered the side to Euro 2016 after a qualifying campaign that featured a 1-0 defeat of Germany before beating Bosnia-Herzegovina in a play-off.
A famous win against Italy at the European Championship secured a last 16 match against tournament hosts France but they were beaten 2-1 in Lyon.
Veteran players Robbie Keane and Shay Given retired after the end of the tournament but the Republic made a promising start to their 2018 World Cup qualifiers before a scoreless draw against Wales triggered a run of four competitive fixtures without a win and the heavy play-off loss to the Danes, which ended their prospects of qualifying for Russia.
"I will not forget the night in Lille when we beat Italy to secure the nation's best ever achievement in the Euros competition," reflected O'Neill.
"The terrific night we beat the World Champions Germany at home in Dublin, or James McClean's goal in that euphoric win against Wales in Cardiff that put us through to the play-offs for this year's World Cup."
In his 55 games in charge, O'Neill had a win ratio of just over 34% with 19 wins, 20 draws and 16 defeats and he used a total of 65 players, including 28 international debutants.
The 66-year-old handed international debuts to 12 players this year as the former Leicester, Sunderland and Celtic manager looked to build a new-look squad for the start of the Euro 2020 qualifying campaign.
The Republic will be among the third seeds for the European qualifiers when the draw is made in Dublin on 2 December.
McCarthy and Kenny among possible contenders
Former Republic manager Mick McCarthy is already being linked with the job and, perhaps significantly, is available following his departure from Ipswich Town in April.
McCarthy guided the Irish to the 2002 World Cup finals and despite his infamous bust-up with Roy Keane on the eve of that tournament, remains highly regarded by the FAI.
Even before O'Neill's departure, there were voices in the Irish media saying that Dundalk manager Stephen Kenny should be next in line following his successes with the League of Ireland champions.
Kenny guided Dundalk to the domestic double this year and his side have won the league title in four of his five seasons in charge.
Brighton boss Chris Hughton, who won 53 caps during his Republic career, is also high in the betting but the FAI could face a difficult task in prising him away from the Premier League club.