Picture the scene. Ahern is sitting in his office in Drumcondra, organising party strategy, devising policies that might be promoted to attract inward investment, hatching plans to get back into government. In walks the flunky. 'Cup of tea, Bertie?' 'Yes please, and while you're at it, would you ever take this big bag of cash and nip down to the AIB to buy sterling with it. And sure, keep a few bob for yourself. Nobody is counting, there's loads more where it came from.'

"I could have done it in instalments, or in different banks, or got somebody else to do it, " the Taoiseach told the Mahon tribunal on Thursday. Instalments? We got a busy flunky on our hands. 'Listen, would you buy five grand of sterling with this today? And you might nip over to Bank of Ireland next Tuesday for another five. We'll have further instalments next week, you'd never know how suspicious it might look otherwise."

Maybe he's telling the truth. Or maybe there was no changing into sterling. Maybe somebody just dropped two more packets of cash into his lap.

So it went in Bertie's Wild Years, circa 199395. He was back in the Castle last week. He will return again tomorrow, and again, into the future. To his supporters, this represents the hounding of an innocent man. His party colleagues make similar noises. Yet who among them actually believes him?

Who among them can say with a straight face that honest Bertie has told the truth about his cash-rich lifestyle? He has given various bizarre versions to the Dail, to the Irish people, in sworn testimony. Is there really anybody out there, familiar with the barest detail, who believes he is telling the truth? And if nobody believes him, are we to take it that he is deemed by popular acclaim to be above the law?

There are two processes afoot at the moment in relation to Bertie Ahern and the stacks of cash that found their way into his bank accounts. One involves establishing facts. The other is concerned with perception, as expressed by public opinion.

Let's look first at the boring and complicated business of the facts. In Dublin Castle, the Mahon tribunal is attempting to get to the truth. The process began with an allegation that Ahern took bribes. The first step in this inquiry was to check out whether there may be validity to the claims. This involved examining the Taoiseach's finances.

The examination threw up huge cash lodgements to his accounts, way beyond his salary.

This raised suspicions that could have been quelled by Ahern. It was in his interest to speedily whack the suspicions on the head. He could have provided a detailed account of where he got the money, who gave it to him, and why. He chose not to.

For two-and-a-half years, the tribunal had to keep probing him in correspondence as to the origins of the money. He wasn't forthcoming. In February 2005, he swore an affidavit that was inaccurate, to say the least. Ahern is a leading politician, an intelligent, astute man, with the best legal brains at his disposal, and he is now claiming ignorance about a basic legal obligation.

It was only earlier this year, largely through pressure brought about by its own enquiries, that the tribunal discovered most of Ahern's foreign exchange transactions, principally the stg�68,000 which is currently the subject of public hearings.

In 2006, Ahern retained accountant Des Peelo to compile a report on his finances. It is now clear, and the Taoiseach accepts, that Peelo was not furnished with the full facts.

When cooperation isn't forthcoming, it is standard practice to invite the subject of an inquiry to a private interview. This took place last April. In the course of the interview, more details emerged.

When answers are still not satisfactory, the next step is to call a public hearing into the matter. All of this could have been avoided if Ahern had come clean at the earliest stage.

That he chose not to is baffling, particularly if his narrative is as innocent as he makes it out to be. He could have kept his bulging bank accounts away from the public gaze. He could have summarily dispelled all allegations that he received bribes.

Three days of public evidence have illuminated a few things. Ahern's story has changed as much as that of his nemesis, Tom Gilmartin. His journey from examination of his accounts to public hearing dispels the cock and bull he propagated about being desperate to help the tribunal. The bizarre explanations he gave to the public now appear more bizarre, even nonsensical.

The holes in his story, the ropey explanations, the failure to actively cooperate, are all now established. Any reasonable observer would have to conclude that his story is not believable. The old questions loom larger than ever: who exactly gave him the money? Why?

Is he telling a pack of lies, to the Dail, to the public, under oath?

The tribunal is hamstrung in one regard. Its modus operandi is to follow the money. In this case, the trace backwards stops at lodgements to Ahern's accounts. Nearly all lodgements were made in cash and therefore can't be traced back to origins. Either this is fortunate for Ahern in that the trail goes cold, or it is unfortunate because it hinders him in standing up his story.

Beyond the inquiry, the battle for public opinion rages. Here, established facts are ignored and a new reality created in a manner that would leave George Bush green with envy.

In this world, the unpopular and costheavy tribunal is hounding Ahern. His personality and achievements are wheeled out as proof of his innocence.

The termination of his marriage is cited as reason for his bizarre financial shenanigans.

By 1994, his marriage had been over for seven years, and for most of that time, he was in another long-term relationship. His bank details do not support the contention that he was strapped for cash during this period. The "personal circumstances" about which he told the Dail last October were not as he portrayed them at the time.

Elsewhere, public life is taking a battering. A slew of government ministers has come out in support of the Taoiseach. The effect has been to pitch the inquiry into Ahern's finances as a battle between the tribunal and the government. If these politicians, presumably in full command of their critical faculties, really believe their leader's story, we have a serious problem. If they don't, we have an even bigger problem.

Look at Brian Cowen, a Fianna Failer to his fingertips, a self-proclaimed Republican in the broadest sense of the term. We are to take it that he is happy to defend a party leader who dealt in suitcases stuffed with cash and envelopes from Manchester bulging with �50 notes for Ahern's personal use? And that's just the innocent version of the Taoiseach's carry-on.

Last Sunday, Willie O'Dea issued his own tribunal report into the affair. He accused Mahon of "acting outside the law". He drew conclusions from Ahern's evidence that were for the birds. Afterwards, he stayed schtum about his startling assertions, as did his cabinet colleagues. But the message went out. The tribunal was undermined. A blow was struck in the PR battle.

On Wednesday, in an unseemly joining of the battle, Feargus Flood said O'Dea was wrong. Flood is retired and has no jurisdiction to make such a claim.

If the government believes the tribunal is acting outside the law, a debate should take place on its future when the Dail reconvenes next Wednesday. If Willie was just shooting off at the mouth, and if we are living in a functioning democracy, he should get his P45.

Flood also said no impropriety on Ahern's part had been uncovered thus far. That evening, the government press office emailed RTE with the good news, as if it was a jobs announcement, or a positive opinion poll. What the hell is going on here?

The executive is pitting itself against an instrument of the Oireachtas set up to investigate corruption in politics. All this, just to protect a leader who does not appear to be telling the truth. Close your eyes and you might be in Putin's Russia, or even Chavez's Venezuela.

We have reached a grubby juncture, as the Taoiseach goes in and out of Dublin Castle, trailing the various versions of his story behind him.