The vastly-experienced midfielder had unexpectedly left Manchester United by mutual consent a little under a month earlier, with his relations at Old Trafford having come under increasing strain.
As a free agent, the 34-year-old had been linked with several clubs in England and abroad, with David Beckham urging Real Madrid to consider signing the Irishman.
But instead Keane chose to join the club he had supported as a boy, fulfilling an ambition he had voiced several years earlier to end his career at Celtic Park.
"I gave other clubs a chance and was impressed with them all. But it's a great move for me. I feel that this is where I belong and I'm here to work hard and win games," said Keane.
"I've been in the Premiership for a long time and feel I have proved myself there and needed a different environment, different teams, different grounds."
Celtic landed themselves a midfielder who had been part of seven Premiership titles, four FA Cups and of course the Champions League win with Manchester United.
He brought with him a reputation as a fiercely combative enforcer, and his enthusiasm appeared undimmed – something his new team-mates could attest to as he was soon seen criticising what he did not like during matches.
There was plenty to criticise as his debut ended in a shock Scottish Cup defeat to Clyde, but Keane would go on to win the Scottish Premier League and League Cup with Celtic that season.
However, they would not see the best of Keane during his time in Glasgow as he suffered with a hip injury.
He was limited to just 13 appearances in all competitions before announcing his retirement in June 2006, a year before his contract had been due to end.