The writing was on the wall for a long time. Long before Cyrus Christie put the ball into his own net. Long before Christian Eriksen's hat-trick for Denmark sealed their progress to the World Cup.
Back in March, Ireland entered their home game with Wales in control of Group D, having beaten Austria away and taken a point from Serbia in Belgrade. Both were impressive results that disguised inadequate performances from a team managed by a pragmatist who believes - rightly - that football is a results business.
Against Wales, Ireland suffered a huge blow as captain Seamus Coleman had his leg broken by defender Neil Taylor. The Boys in Green played 20 minutes against 10 men in Dublin, but failed to muster any chances of note despite the man advantage. From there, the Road to Russia was in ruins.
Ireland's performances under O'Neill have long been disappointing, with the former Celtic and Leicester City boss regularly batting off criticism of his anti-football by pointing to the table and citing his qualification for the expanded 24-team European Championship. The Boys in Green boss pointed to the technical limitations of his players, as if a side made up of largely Premier League talents could not string more than two passes together.
O'Neill never had a Plan B, hoping that his 'rope-a-dope' approach of sitting back, defending resolutely, tiring the opposition and hoping for a mistake would get him the points he needed. Ireland's goals came from set-pieces or defensive errors, which luckily fell Ireland's way time and again even as they stifled opponents with atrocious un-entertainment.
More than once, Ireland played against 10 men, but failed to make it count in another crucial qualifier, losing 1-0 at home to Serbia despite having a man advantage for much of the second half. Again, the players seemed not to know what to do as their manager had nothing in reserve.
However, somehow, a victory against Wales in Cardiff saw Ireland sneak into the playoffs. The script was the same, Ireland retreated, retreated, retreated and amazingly defeated the Welsh thanks to another defensive mistake.
Having suffered 90 minutes of Ireland's anti-football on Saturday, Denmark knew it would likely take just one goal in Dublin to send them to Russia. Their manager Age Hareide said as much before the game: "We know that one goal will get us there."
Ireland were never going to score twice under O'Neill. He just does not know how to set his team up to attack the opposition. So, even though Ireland scored first through Shane Duffy from a set-piece, once Denmark were level through uncharacteristic poor defending from the Boys in Green, it was over. The Christian Eriksen hat-trick was just icing on the cake for the Danes and salt in the wounds of a deflated Irish side.
Finally, O'Neill's luck had run out. Without that, there was nothing left for his players to work with. Having played give-away football for so long, they never knew what to do with it to try to salvage their World Cup hopes. Denmark, on the other hand, known for their directness, knew exactly what to expect from Ireland and made them pay.
Bizarrely, before the play-offs the Football Association of Ireland had confirmed a contract extension for O'Neill. Like the man in charge of their national team, they had no Plan B. But the indicators were there all along, Plan A was the wrong one.
Like so many teams not going to the World Cup, Ireland should be looking at trying to regroup and rebuild, but with O'Neill in charge, they are stuck in the past and going nowhere fast.