Keane slotted home in injury time to give the Irish side victory in Celtic captain Jackie McNamara's testimonial.
The game was also Paul Lambert's last in a Celtic shirt, before he takes over as manager of Livingston.
Others including McNamara, Bobo Balde, and Rab Douglas may also have been playing for Celtic for the final time.
With Celtic having won the Scottish Cup just 23 hours previously, the lack of urgency about the home side's play was unsurprising.
Kevin Kilbane stabbed the ball wide from close range before Craig Beattie forced a save from Shay Given at the other end.
Craig Bellamy had a well-taken finish chalked off by an offside flag and Douglas, who is set to join Leicester, denied Sunderland striker Stephen Elliott with a smart block.
Beattie was far too slow, after a run by Bellamy had taken out Given, and Steven Reid was able to nip in before he could locate the empty net.
Shaun Maloney came on for Chris Sutton just after the half-hour mark and forced a tip-over from Given with a well-aimed free-kick.
The afternoon needed spicing up and the arrival of Roy Keane, Robbie Keane and Damian Duff after the interval injected some extra quality to the Ireland line-up.
McNamara was replaced to cheers by Didier Agathe in the 57th minute and two minutes later, Henrik Larsson made his return to the Celtic line-up.
Fellow old boy Lubo Moravcik also came into the action and Aiden McGeady, who had played for Celtic in the first half, reappeared for Ireland with 20 minutes remaining.
Magnus Hedman, on for Douglas' replacement David Marshall, denied Republic substitute Matt Holland with a block from eight yards, while at the other end. Joe Murphy, Ireland's third goalkeeper of the day, denied Momo Sylla.
Lambert bade his farewell with 10 minutes remaining and the game seemed destined to remain scoreless until Robbie Keane slotted a cross from Gary Docherty into the net in stoppage time.
After the final whistle, O'Neill paid tribute to McNamara before bidding farewell to the fans who had sung his name.
Celtic favourites Lubo Moracivik and Simon Donnelly also made appearences.
Celtic: Douglas; McNamara, Varga, Balde, Laursen; Beattie, Lambert, Lennon, McGeady; Sutton, Bellamy.
Subs: McManus for Varga, Maloney for Sutton, Wallace for Bellamy, Marshall for Douglas (all 36 mins), Fernandez for McGeady (half time), Agathe for McNamara (57 mins), Larsson for Beattie (59 mins), Sylla for Lennon (60 mins), Hendman for Marshall (61 mins), Moravcik for Wallace (69 mins), Donnelly for Lambert (80 mins), Wallace for Sylla (88 mins)
Brian Kerr Republic of Ireland XI: Given; S Reid, Cunningham, Doherty, Carr; Quinn, Miller, Kilbane, A Reid; Elliott, Morrison.
Subs: Kenny for Given, O’Brien for Cunningham, Kavanagh for Kilbane, Roy Keane for Miller, Holland for Quinn, Duff for A Reid, Robbie Keane for Morrison (all half time), Murphy for Kenny, McCarthy for Carr, McGeady for Elliott (all 69 mins), Carr for S Reid (80 mins).
*Courtesy of BBC