Ireland's World Cup dreams unravelled in the most appalling manner last night at the Aviva Stadium in Dublin. In the end, a relatively average Denmark side stuffed us 5-1. Bizarrely, we can hang onto the notion that it might all have been so different. But even to suggest that is akin to hiding from the grim truth.

Over the 90 minutes, it is fair to say that Ireland were a shambles. And while the players must shoulder their share of the blame, to differing degrees, ultimately it is Martin O'Neill who must carry the can for what was one of the worst performances by an Irish side in living memory.

There were accidents along the way and unforced errors. But this was not a result that happened by accident. It happened as a result of the extraordinary, inexplicable, and too often completely illogical decisions made by the manager – in team selection, and in particular, in this case, in substitutions. Ireland were completely undone by the folly of the man in charge.

My own response to the team-sheet was to groan. Both Wes Hoolahan and Shane Long should have started.

In Hot Press, we had called it right in every respect but one. We knew that he would bring David Meyler back in for Callum O'Dowda. We knew that he would leave Wes Hoolahan on the bench. But we thought he would bring Shane Long in for Daryl Murphy. In that we were mistaken.

In fairness to Murphy, he worked like a demon and did a few good things during the first half. But there is little doubt that Shane Long's greater pace would have asked more questions of the Danish defence.

The game developed in a peculiar, stuttering manner. Instead of playing wide where he is best, McClean seemed to be operating as a second striker. The effect, however, was that no one seemed quite sure where they were supposed to play. Jeff Hendrick in particular seemed dazed and confused.

Denmark had most of the possession and forced a few good saves from Darren Randolph. But a Danish mistake, from a Robbie Brady free, gifted an opportunity to Shane Duffy, and the Derryman wanted it more than anyone else. He leapt superbly, getting his head to the ball ahead of Kasper Schmeichel. Goal!

To an extent it was against the run of the play, but that didn't matter. In the aftermath, Ireland started to stitch a few moves together and for a short while it looked as if we might just have it in us to pull away.

First Cyrus Christie put a lovely ball in, which Daryl Murphy flicked superbly goal-wards, only to see it hit the side-netting. Another fine move saw James McClean (pictured) in on goal, but he pulled his shot just wide. Had the other Derryman put that away, it would have been 2-0 – and who knows how it would have ended?

Instead, at the other end, the Danes scored two in quick succession. The first goal was decidedly unlucky, Cyrus Christie turning the ball into his own net, after Christiansen's scuffed shot rebounded off the post. The second goal also came about as a result of an individual error, Stephen Ward gifting the ball to the Danes. Looking to cover two players, Shane Duffy moved towards the ball, leaving the Danish master craftsman in space. Christian Eriksen made no mistake.

It was 2-1 at half-time. The situation was retrievable. But, like a drunken sailor gambling all, Martin O'Neill decided on not one, but two changes.

He was right to bring Wes Hoolahan on – though one might ask, if he was potentially the answer now, should he not have been in the thick of it from the start? Stephen Ward should probably have been the one to make way. Robbie Brady could have dropped in to full-back. The spine of the team would have remained intact. And the plan should then have been to get Shane Long on, as soon as possible after the new shape had settled.

But Martin O'Neill rolled the dice. He brought Aiden McGeady in, as well as Wes Hoolahan, withdrawing both the captain David Meyler and Harry Arter – crucially weakening the central midfield area. We paid the price in rampant Danish looting.

Under Hoolahan's influence, Ireland started to weave a few passes together. But the defensive strength of Meyler and Arter was gone. And as a result, we were easily over-run. Denmark got a third. And a fourth, this one again from a Ward mistake.

It was end to end stuff, but the only ones who ever really looked like scoring were the Danes.

There was one final bizarre decision by O'Neill, when he brought Shane Long on – for Ciaran Clarke. Our shape was gone. We were rendered stupidly vulnerable and it cost us badly.

The real indictment of Martin O'Neill's decision-making is that Aiden McGeady contributed so little. Don't get me wrong. He is a decent footballer. I like him. But it was pure folly to bring him in, and risk all, on the possibility that he might conjure something special. Well, what happened is that we were left defensively bereft.

It was an act of grand madness, for which there is neither rhyme, reason nor excuse. Why, for example, was Callum O'Dowda more suited to the game in Copenhagen than Aiden McGeady? And if he was, then why not follow through on that belief and bring him in again, rather than the Sunderland winger?

The answer is that O'Neill was operating on some mystical notion that if he put McGeady on the pitch, his old Celtic buddy would deliver. It was an act of 'faith'. But there is no God. The move failed spectacularly.

A fifth Danish goal duly arrived from the penalty spot, stuck away by Bendtner in the 90th minute. Ireland really were humiliated.

But what of the men who had been taken off at half-time? How were Harry After and David Meyler feeling after watching those final 45 crazy minutes from the stand?

It was all just too bizarre.

The reality is that Martin O'Neill threw away whatever chance we had at half-time. Our dreams were blown to smithereens by his substitutions. It is as simple as that.

In the context, it is hard to justify extending his contract. We screwed up against Georgia. We lost unnecessarily to Serbia in Dublin. From a position of strength in the group, we slipped back into the mire.

And we played agricultural football at almost every stage. We did well to snatch what was a lucky victory in Wales. We worked hard for it. Fine. But in the end, Martin O'Neill's management came up not just short, but way short.

At every stage of the campaign, there was a complete absence of logic in what Martin O'Neill was doing. He was playing hunches. Sticking the captain's armband on people too easily. Giving individuals a run as if surprising the opposition was sufficient virtue to justify it.

Leaving Wes Hoolahan on the bench, when Ireland generally play far better with him on the pitch, and score more often.

Tonight, at half-time, it was as if he didn't really want to be proved wrong about Wes. Bringing Hoolahan on would have been enough at that juncture. Ireland still had 45 minutes to score two goals. Denmark, it is worth noting, ended up scoring three – so it was doable. Was it to guard against 'I told you so's' – if bringing Wes on worked – that he added McGeady, to confuse the issue?

But how mad was it to remove two of your three central midfielders at half time, for two patently more light-weight players?

It was footballing hari-kiri. It was suicidal. And now we are dead.

The bottom line is that Martin O'Neill should now fall on his sword. That would be the dignified, warrior-like thing to do.

Ah well. We have lots of friendlies to look forward to. Wonderful. The dream is over, for another four years. Now that is hard to swallow. Blame the players by all means, if that is your angle. The fact is that they ran their guts out – the likes of McClean and Duffy in particular. The cliché is 'we weren't good enough'.

Perhaps. But we will never know for sure. The manager made crazy decisions. That, we can no longer ignore...