Republic of Ireland assistant manager Roy Keane called police to the team's hotel following an incident with a member of the public, the Football Association of Ireland has said.
An ambulance also attended the Portmarnock Hotel, near Dublin.
A fan is believed to have confronted Keane in the hotel's reception area, tearing up a book after a heated argument turned physical.
The FAI says Keane has its "full support" after "inaccurate reports".
It is believed the area where the incident happened is covered by closed-circuit security cameras.
The Republic of Ireland team are staying at the hotel before Friday's Euro 2016 qualifier against Scotland at Celtic Park.
"We wish to clarify that Roy Keane called the Gardai following the incident," an FAI statement read.
"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."
|Republic of Ireland manager Martin O'Neill|
|"It's only a distraction, that's all it was. But I think we are fine, I think the players are fine, I think the man in question is fine too and we are ready to go."|
A police spokesman said: "Gardai are investigating an incident which occurred at a hotel in Portmarnock in the late afternoon."
They added no arrest was made and and no complaint was received.
Former Manchester United midfielder Keane, 43, won 67 Republic of Ireland caps between 1991 and 2005.
In November 2013, the ex-Sunderland and Ipswich manager was appointed assistant boss to O'Neill, a role he combines with the same job at Aston Villa.
At his pre-match media conference, O'Neill said: "It's only a distraction, that's all it was.
"I made a statement earlier on today and there's not really much more I can talk about.
"But I think we are fine, I think the players are fine, I think the man in question is fine too and we are ready to go."
Quizzed further about his number two, O'Neill replied: "He's okay - he's actually here with us now, so if we could end the press conference right now it would be great because you will catch him."