The hardcore supporters who call themselves the 'Jungle Bhoys' after the old Parkhead terrace, were determined to make the Corkman feel at home, holding up green, white and orange cards to transform the entire Lisbon Lions stand into one giant tricolour. It was a theme reflected all around the ground in the abundance of crisp, new Irish flags bearing the legends 'There's only one Keano' and 'A Celt At Last', many of them waved by the big contingent from the old country who'd filled every last plane seat to Glasgow for a sell-out game which saw Celtic Park filled to its 60,000 capacity.
Of course, Keane had actually been an active Celt last week, but at this most tradition-conscious of football clubs there was almost a sense that the faithful would prefer to erase Clyde's shock cup victory from the official history. After all, the argument ran, this was the real debut - Keane's first appearance in the actual hooped shirt.
On Saturday, the burning question before kick-off was of the mathematical kind. Since three into two won't go, fans and media alike pondered how Gordon Strachan would solve the problem of accommodating Roy Keane, Neil Lennon and the returning Stilian Petrov in his midfield. The solution turned out to be that he wouldn't even try: Keane would make his home debut as a centre-half, replacing the hapless Du Wei to partner Stephen McManus.
And for the former Manchester United man, the game couldn't have gotten off to a better - or less demanding - start. In fact, he hadn't even had a touch of the ball by the time his side were one-up, the wonderfully gifted Shunsuke Nakamura sweeping an exquisite free-kick into the top corner of the Kilmarnock goal to give Celtic the lead inside the first three minutes.
Twelve minutes later, Celtic were awarded a penalty, and the chants of 'Keano, Keano' rolled around the ground, as the fans begged for their latest hero to crown his home debut with a goal. But Keane simply remained watching from the other end as Shaun Maloney converted.
Now, the good times were definitely back, as Celtic Park purred to the sight of the Hoops dazzling their opponents with fast, fluent one-touch passing, an at times sensational display of attacking football in which the blistering pace of Maloney and the trickery and vision of man of the match Nakamura were the outstanding elements.
If Strachan could have called upon a fleet-footed striker instead of the increasingly pedestrian John Hartson, Celtic might have been four or five up - before, suddenly, it all started to go wrong.
McManus was the first culprit, tripping Colin Nish to concede the penalty which allowed Steven Naysmith smuggle Kilmarnock back into the tie. And then six minutes after the restart came something of an unwanted collector's item - an elemental error by His Corkness which helped set up an unlikely equaliser.
There are many things you wouldn't expect to see Keane doing - being taken in by a fake sheikh, say, or pretending to be a cat on Celebrity Big Brother. Almost as unthinkable is that he would be caught in possession close to his own byline. But it happened as the Irishman dithered when he could have cleared, and was then robbed of possession by Nish, who laid the ball off to Garry Hay on the left wing. And it was from his cross that Danny Invincibile rose to head Killie level.
It doesn't require too much effort to imagine what Keane's response would have been if a Manchester United team-mate had displayed a similar lack of concentration. But since a man can't really give himself a good bollocking in public, Keane opted to let his feet do the talking.
And the response was immediate: Celtic went straight back up the other end and, after goalkeeper Alan Combe failed to hold Keane's low shot, McManus reacted quickest in the six yard box to poke the home side back in front. It was a case of Paradise lost, Paradise regained.
With 24 minutes left, Maciej Zurawksi made it 4-2 for Celtic, but the chants of 'Easy, Easy' from the Jungle Bhoys were a direct contradiction of the extent to which Celtic had actually made the job difficult for themselves.
THE ONE big blot on his copybook aside, Keane performed solidly enough. True, a couple of fractionally mistimed challenges pointed to a lack of match fitness, but he made at least one vital block to stop a goal-bound shot, contributed a couple of fine, clearing headers and kept his passing from the back simple and effective. However, it will clearly take him time to impose his personality on the side - on Saturday, it was his partner McManus who took the organisational lead, while Keane went about his own business with the minimum of fuss.
Celtic's defensive lapses, of which full-back Paul Telfer was especially guilty, took some of the gloss off the win for manager Gordon Strachan. "We looked like a work of art going forward and like my art at school going back," he said afterwards. "Roy did alright. It's a long time since he's played in that position."
Keane is set to retain his centre-half berth for the time being, but at some point the manager will surely have to bite the bullet and decide if it's the Corkman or Neil Lennon - a player perhaps best described as Keane-lite - who will anchor the Celtic midfield. For Keane's own bedding-in process at the club, you'd think it would also be important that his role be defined sooner rather than later.