Roy Keane was involved in an altercation with a fan in the Ireland team hotel after which the man received medical attention.
The man was taken to hospital following the incident at the Portmarnock Hotel and Golf Links, where the Ireland football team are staying ahead of their crunch match against Scotland.
The Irish Independent can reveal that the man, whom Keane has history with, had a copy of his book and asked the Ireland assistant coach to sign it for him. Keane responded: "I'm not signing your book."
When Keane refused the man - who is believed to have written about the former Manchester United star in the past - began tearing pages out of the book in the reception area of the hotel.
The former Ireland captain is believed to have confronted the man, who then fell to the ground and an ambulance was called.
FAI sources told the Irish Independent last night that the football association was satisfied no punches were thrown, and they have numerous witnesses to the incident.
It is believed that the man is being treated in Beaumont Hospital.
Gardai were called after the alleged altercation.
The FAI said Keane (43) called the gardai and he had their "full support".
The incident happened in the busy reception area of the hotel after training.
Keane has the full backing of the FAI, team manager Martin O'Neill and the FAI CEO John Delaney.
Gardai have received no official complaint in relation to the matter.
"The exact nature of the incident has to be established," a garda source said.
It is understood the man, who was taken to Beaumont Hospital by ambulance yesterday evening, was not seriously injured.
He is believed to be the son-in-law of a well-known entertainer.
It is understood that the incident occurred just outside the bar area.
Ireland striker and captain Robbie Keane, who was sitting in the hotel lobby with two people last night, refused to comment when approached at the hotel.
"I'm with the family... I haven't seen them for a month or so. Thank you," he told the Irish Independent.
Management at the hotel also refused to comment.
Keane and the rest of the squad are due to fly to Glasgow this afternoon following a morning training session.
It is understood that gardai attended the scene after being called. However, no criminal complaint was made.
"This matter is being investigated by gardai but a complaint must be received for further action to be considered in the case," a senior source said.
"And there has been no complaint yet."
Gardai from Malahide station were interviewing witnesses to establish the exact circumstances last night.
The incident could prove disruptive in the build-up to the Euro 2016 qualifier against Scotland tomorrow night.
The team has been staying in the Portmarnock hotel this week as the squad trains in Malahide.
"Following inaccurate media reports which refer to an incident involving a member of the public this evening at the Republic of Ireland team hotel, we wish to clarify that Roy Keane called the Gardai following the incident. The manager Martin O'Neill and the FAI are fully aware of the circumstances surrounding this event, and the Assistant Manager has their full support," the FAI statement said.
The incident has overshadowed the build-up to the team's crucial Euro 2016 qualifier in Glasgow.
Ireland lead Scotland by three points after three games in Group D.
With Germany and Poland also jostling for position in the group, this game is likely to prove crucial.
Keane was appointed assistant manager of Ireland in November last year as part of the so-called "dream team" partnership with O'Neill.
The pair are returning to their old stomping ground at Celtic Park for the clash.
Former Manchester United skipper Keane made headlines recently with the release of his controversial autobiography 'The Second Half'.
In it, he criticises former team-mates and colleagues, as well as stoking the flames of his ongoing war of words with his long-time boss at United, Alex Ferguson.
In the book, Keane recalls the upset he felt after he was sacked as manager of Ipswich in January 2011.
Keane outlined in the book how he failed in his second stint as a manager and that turning to his role as a television pundit, as he did for ITV, further compounded his feeling that he had ultimately failed.
Steven Naismith has suspended his friendship with Aiden McGeady and James McCarthy until the dust settles on Scotland's Euro 2016 match with the Republic of Ireland tomorrow - but the Scottish striker defended his Everton colleagues' decision to choose Irish jerseys over the dark blue shirts they were also entitled to wear.