Stephen Pearson, 23, a Scotland international, appears on the grainy clip, which is believed to have been filmed on a mobile phone camera. His agent has confirmed his attendance at the event but denied any sectarian chants by the player. Other Celtic players also reportedly attended the event, which happened in Ireland recently.
The clip shows a rendition of the Irish folk tune, "Fields of Athenry", punctuated by shouts of "IRA" and "Sinn Fein". Pearson's agent, Willie McKay, said: "Stephen's seen the video and he said he sang 'Fields of Athenry', but he never shouted 'IRA'. This song is sung at Celtic games and at supporters' nights every week, there's nothing sectarian about the 'Fields of Athenry'. I can assure you that Stephen is not sectarian at all."
The agent of another prominent Celtic international player who was reportedly involved dismissed those reports as "complete rubbish".
The vice-chairman of Celtic's Old Firm rivals Rangers, Donald Findlay, resigned his post in 1999 after he was filmed singing a sectarian song at a social club.
Celtic's manager, Gordon Strachan, said of the video: "I have not seen it. I have no comment." He preferred to concentrate on tomorrow's match. "He [Keane] will definitely play a part," Strachan said, adding that "midfield's the place for him". There had been speculation about a defensive role but with Stilian Petrov out injured, Keane seems likely to start in a central position.
Keane has not played a match since 18 September, when Manchester United drew 0-0 at Liverpool, and he has not experienced a competitive win since United won 2-0 at Newcastle on 28 August. Barring one of the biggest Cup upsets in memory, that should change tomorrow.
Rangers will be looking to avoid any embarrassing slip-ups this afternoon at Ibrox against Second Division Peterhead. Hearts versus Kilmarnock is the pick of the other ties, while Gretna of the Second Division will fancy their chances at First Division St Johnstone. Spartans, who play in the East of Scotland League and are the lowliest club still in the competition, host Third Division Queen's Park.