Suddenly the spectators were reminded that this occasion was something other than the pretext for a celebration of national unity. It was a showdown between the two teams who, more than six months away from the World Cup, both fancy themselves as Europe's best chance to keep the All Blacks' hands off the Webb Ellis Cup. Inside the whitewashed lines, sentiment was never a factor.
Until Vincent Clerc smashed through the Irish cover to touch down beside the posts with barely a minute left, however, it was tempting to wonder whether the Irish Rugby Football Union would be thinking about inquiring about the possibility of making Croke Park its permanent home. As their players fought their way to a narrow lead in the second half, the thousands standing on the open terrace of Hill 16 struck up a chorus of The Fields of Athenry, immediately finding an echo around the great horseshoe of the stadium.
It was a beautiful and a thrilling sound, in a majestic stadium, and it made it seem as though Irish rugby might just have found a new home. What, after all, could provide a more optimistic sign of the healing of Ireland's ancient rifts than a complete rapprochement between the Gaelic sports on the one hand and the Anglo games on the other? That Gaelic football, hurling, rugby union and football should share a single address - and one so rich in cultural meaning - would be a symbol worth more than all the politicians' pieces of paper.
But then, less than a minute after Ronan O'Gara had stroked over the penalty that widened the lead to 17-13, the Irish defence switched off and opened the way for that final, gut-wrenching French attack, Lionel Beauxis's conversion merely sealing the margin. The sound of a nation expressing its joy was cut off as if by a switch, leaving only the noise of a few thousand delirious French supporters, perhaps surprised to discover that their players were capable of such a display of resilience. Goodness knows what Ireland's rugby followers will think of Croke Park after this. Lansdowne Road was famous for winds that blew from all points of the compass to confound the most dead-eyed of kickers and freeze the bones of spectators insulated by wool and whisky. The sudden gust that swept through Croke Park, however, imparted an even more bitter chill.
Now Ireland's second game in their adopted home, a meeting with the world champions a week on Saturday, takes on the most urgent significance, which has little to do with the outcome of the 2007 Six Nations Championship. Pride is at stake, and the restoration of the momentum that Ireland had hoped to maintain all the way to the start of the World Cup finals in September. But an English victory in front of another 81,000 crowd, following on yesterday's calamity, might even be enough of a double whammy to send Ireland scuttling back to Dublin's south side.
It could happen, at least in practical terms, since not a single brick of Lansdowne Road has been removed. When the Gaelic Athletic Association, at its annual congress in 2005, agreed to set aside its cherished Rule 42, prohibiting the playing of "foreign games", it was on the basis that the change would be "for the duration of any redevelopment of Lansdowne Road". At the moment, however, the renovation of the old ground is looking a bit more Wembley than Twickenham, and even that may be erring on the bright side.
In every other respect Croke Park was a perfect setting. The open end provided a view striking enough to evoke a momentary regret that the home of English rugby has closed up its south stand. The floodlights illuminated the dramatic final act, and a widely expressed fear that the turf, a perfect emerald green, would provide too slippery a foothold for rugby was undermined by any number of stirringly effective rolling mauls from both sides.
But if Brian O'Driscoll was cursing the injury that kept him out of this historic match, his regret would only have been redoubled by the nature of the defeat. It is hard to imagine that, moments after O'Gara kicked his 78th-minute penalty, Ireland would have offered such a porous response to Clerc's incursion had the great man been around.
Instead, on the ground where Muhammad Ali took 11 rounds to dispose of Al "Blue" Lewis back in 1972, Ireland forgot the most basic principle in sport, which is to keep going until the referee blows the final whistle. It was a lesson that cost them just under Â£1m, which is the fee the IRFU is paying the GAA for each match at Croke Park. A convincing victory over England could still make it seem money well spent.