Keane was one of two Ramblers representatives in the inaugural FAI/FAS scheme in the Dublin suburb of Palmerstown, and it was through this initiative that he got his first taste of full-time training, facilitated by living in nearby Leixlip, County Kildare from Monday-Friday.

In an FAI Youth Cup match against Belvedere in February 1990, Keane's performance attracted the attention of watching Nottingham Forest scout Noel McCabe, who asked him to travel over to England for a trial.

Keane initially found life in Nottingham difficult due to the long periods away from his family, and he would often ask the club for a few days' home leave to return to Cork.

Two weeks later, Keane broke his Wembley losing streak by helping United to a 4–0 victory over Chelsea in the FA Cup Final, sealing the club's first-ever "double".

[24][25] Keane received his first red card as a Manchester United player in a 2–0 FA Cup semi-final replay win against Crystal Palace, after stamping on Gareth Southgate,[26] and was suspended for three matches and fined £5,000.

[44] He initially received a three-match suspension and a £5,000 fine from The Football Association (FA), but further punishment was to follow after the release of Keane's autobiography in August 2002, in which he stated that he intended "to hurt" Haaland.

After the defeat, Keane blamed United's loss of form on some of his teammates' fixation with wealth, claiming that they had "forgot about the game, lost the hunger that got you the Rolex, the cars, the mansion".

[11] Earlier in the season, Keane had publicly advocated the breakup of the treble-winning team[51] as he believed the team-mates who had played in United's victorious 1999 Champions League final no longer had the motivation to work as hard.

[52] In August 2002, Keane was fined £150,000 by Sir Alex Ferguson and suspended for three matches for elbowing Sunderland's Jason McAteer, and this was compounded by an added five-match suspension for the controversial comments about Haaland.

[5] Keane unexpectedly left Manchester United by mutual consent on 18 November 2005,[60] during a protracted absence from the team due to an injury sustained in his last competitive game for the club,[61] caused by a robust challenge from Luis García against Liverpool.

His departure marked the climax of increasing tensions between Keane and the United management and players since the club's pre-season training camp in Portugal when he argued with Ferguson over the quality of the set-up at the resort.

Another of Keane's appearances on MUTV provoked more controversy, when, after a 4–1 defeat at the hands of Middlesbrough in early November, he criticised the performances of John O'Shea, Alan Smith, Kieran Richardson and Darren Fletcher.

[65] Keane, in an interview with the Irish media company, Off the Ball, in September 2019, stated that Manchester United were pushing to get him out of the club because he was getting old and his strained relationship with then assistant manager Carlos Queiroz and later on with Sir Alex Ferguson, rather than the mere MUTV incident.

[78] When called up for his first game at the international level, an under-21s match against Turkey in 1991, Keane took an immediate dislike to the organisation and preparation surrounding the Irish team, later describing the set-up as "a bit of a joke".

Keane declared his unavailability to travel with the Irish squad to Algeria, but was surprised when manager Jack Charlton told him that he would never play for Ireland again if he refused to join up with his compatriots.

[11] Despite this threat, Keane chose to stay at home on the insistence of Nottingham Forest manager Brian Clough, and was pleased when a year later he was called up to the Irish squad for a friendly at Lansdowne Road.

Tensions between the two men peaked during a pre-season tournament in the United States when Charlton berated Keane for returning home late after a drinking session with Steve Staunton.

[79] After a row with goalkeeping coach Packie Bonner and Alan Kelly Jr. on the second day of training, Keane announced that he was quitting the squad and that he wished to return home to Manchester due to his dissatisfaction with Ireland's preparation.

Keane immediately gave an interview to leading sports journalist Tom Humphries, of the Irish Times newspaper, where he expressed his unhappiness with the facilities in Saipan and listed the events and concerns which had led him to leave the team temporarily.

"[42][80] Niall Quinn observed in his autobiography that "Roy Keane's 10-minute oration [against Mick McCarthy, above] ... was clinical, fierce, earth-shattering to the person on the end of it and it ultimately caused a huge controversy in Irish society."

The two men, publicly at least, were on opposing sides during the fall-out from the Saipan incident, but they were on good terms at the time of the managerial appointment, with Quinn urging Sunderland fans to "support and enjoy one of football's true greats".

[89] Keane signed a three-year deal immediately after Sunderland's victory over West Bromwich Albion on 28 August, the Mackems' first win of the 2006–07 season after a dreadful run of four consecutive defeats under Quinn's temporary management.

The most notable signings were Keane's former Manchester United teammates Dwight Yorke[90] and Liam Miller,[91] supported by former Celtic colleagues Ross Wallace and Stanislav Varga, [92] as well as Wigan Athletic pair Graham Kavanagh and David Connolly.

Five further players were signed during the January 2007 transfer window, three (Anthony Stokes, Carlos Edwards and Stern John) on permanent contracts and two (Jonny Evans and Danny Simpson) on loan from Manchester United, Keane's old club.

[100] Despite some positive performances, including the historic 2–1 home victory against local rivals Newcastle United on 25 October (the first time the club had accomplished this in 28 years),[101] as well as good showings by recent signings like Djibril Cissé and Anton Ferdinand, the team's general form remained inconsistent.

He signed goalkeeper Márton Fülöp, midfielders Carlos Edwards and Grant Leadbitter and brought in Jack Colback, David Healy and Daryl Murphy on loan to the club.

Usually operating in either a holding or box-to-box role in the centre of the pitch, his most prominent traits were his stamina, intelligence, positional sense, tenacity, aggression, physical strength, and ball-winning abilities, although he was a complete midfielder, who possessed a wide range of skills; indeed, he was also capable of carrying the ball forward effectively after obtaining possession, and either distributing it to other players, controlling the game and dictating the tempo in midfield, starting attacking plays, or even creating chances for his teammates, courtesy of his composure on the ball, first touch, and precise, efficient passing.

"[137] Regarding Keane's complex character, despite his intensity on the pitch, Sean O'Hagan of The Guardian wrote in 2002 that he is "...a committed and confident warrior on the field, a shy, socially awkward, and often lonely introvert off it.

Along with Harry Redknapp and Gareth Southgate (who had previously been stamped on by Keane during an FA Cup semi-final in 1995, leading to a red card), he was a pundit for ITV's coverage of the Champions League final between Manchester United and Barcelona.

[187] Speaking in Dublin at his annual visit to the Irish Guide Dogs for the Blind, he spoke on the loss affecting him, "Triggs was great and went through a lot with me... you will have me crying in a minute, so be careful.