If the Public Accounts Committees investigation into the DIRT scandal were a Sherlock Holmes story, it would have to be The Dog That Didn't Bark, with the Revenue Commissioners playing the role of the dog, except the ending would have to be rewritten.
In the Holmes story the silent canine leads the investigator to solve the mystery and finger the criminal. In the PAC's search for the guilty men in the massive evasion of DIRT by the financial institutions, the Revenue's failure to bark is itself the mystery.
And the evidence this week of the men at the top of the Revenue, the chairman, Dermot Quigley and his predecessor, Cathal Mac Domhnaill, has served not to resolve it but to deepen it.
At the heart of the mystery is an instruction issued to tax inspectors in July 1986 by the office of the superintending inspector. At that time the Revenue did not have the power to inspect the fictitious non-resident accounts that sheltered billions of pounds from their eyes. But under the 1986 Finance Act, it did have the power to inspect the forms on which the holders of the accounts declared themselves to be non-residents.
At the very least, the fact that these forms were being scrutinised by the tax authorities might have helped to dispel the air of impunity that encouraged so many people to believe they could get away with wholesale evasion.
The order of July 1986, known as SIM 263 and issued just as the new powers were coming into effect, pointed out that inspectors are entitled to examine declaration forms. But then it immediately added an instruction not to do so: "Further instructions on the examination of declaration forms will issue in due course and inspectors should not call for declaration forms from relevant deposit takers pending receipt of those instructions."
Almost everything about this key order remains obscure. The wording strongly implies that the order is merely temporary and that the examination of the declaration forms will begin shortly, as soon as the superintending inspector has worked out a detailed procedure. But these detailed orders never came. For the next 12 years, the apparently temporary delay became, without anyone even issuing a positive instruction to that effect, a permanent ban on looking at the forms.
And it is not even possible to say who actually drew up this order. At first it was assumed that it came from the superintending inspector Seamus O'Connell. Then Mr O'Connell suggested to the committee that responsibility lay with a colleague, Tony Brown, now dead. Finally, even this attribution collapsed, when the chairman, Dermot Quigley, pointed out that Mr Brown did not work in the relevant section of the Revenue at the time, leaving the mystery intact.
As a clearly frustrated Pat Rabbitte put it to the former chairman of the Revenue Commissioners, Cathal MacDomhnaill, we have progressed as far as yourself, the former chairman, and we still don't know who originated SIM 263.
Now, is that fair to us? That we have been here for eight or nine days and to a simple question about SIM 263, we can't get an answer.
Even more disturbing than the inability of the leaders of the Revenue to say who issued a crucial instruction is their production of directly misleading information.
One example of this is a statement by Mr Quigley in the Comptroller and Auditor General's report on the affair. On page 91 Mr Quigley states that SIM 263 did not prevent inspection in cases by the investigation branch where it came across information which suggested the existence of bogus non-resident accounts. In evidence to the PAC, however, members of the investigation branch have made it clear that this was untrue, and that all the forms were off-limits to them.
This week Mr Quigley, though insisting that he had not set out to mislead the committee, accepted that his understanding might not have been 100 per cent.
Meanwhile, Mr Quigley's predecessor as chairman, Cathal Mac Domhnaill, also turned out to have misled the PAC. In his evidence last year, when the scandal was first being investigated, he said that in 1991, "we approached all the financial institutions on the basis of our own analysis of the involvement in accounts".
This was not merely untrue, it was the direct opposite of the truth. For, as the committee heard this week, it was Mr MacDomhnaill who had decided in 1992 not to approach the financial institutions about their evasion of DIRT. This happened at another key moment in the scandal, the fate of a paper called "Tax Evasion 97 - The Role of Concealed Deposit Accounts" prepared in early 1992 by the Revenue's assistant secretary, Sean Moriarty, on his own initiative.
Here was an officer at a very high level in the Revenue drawing up a detailed consideration of what was at stake.
He had realised, as he spelled out in the document, that the largest and most serious tax evasion centres on facilities to conceal significant wealth created out of profits or income. One method of evasion was the setting-up of offshore companies. The other was the creation of fictitious non-resident addresses for Irish residents.
Sean Moriarty's paper was no mere expression of vague intuitions or broad impressions. It gave a detailed account of striking anomalies, like the fact that close to half of all deposits in the Agricultural Credit Corporation were held in supposedly non-resident accounts.
In case his colleagues did not grasp the point, he spelled it out: the non-resident proportion of ACC accounts was by any standards extraordinary. This is a State bank set up to finance the development of agriculture, with a customer and management base drawn from the Irish farming community. It has no international link-up and is not known to have marketed abroad. In other words, the ACC's vast numbers of rich foreigners were in fact staunch farmers from the Golden Vale and the plains of Meath.
Sean Moriarty made concrete proposals for tackling this kind of evasion. The chief executives of the banks could be called in either individually or in groups and warned that they would have to regularise the bogus accounts immediately. The Revenue could warn the banks that it would be obliged to activate all legal remedies against both the institutions and individual branch managers if they did not stop facilitating the frauds. They could also make it clear through a press statement that the people opening the accounts would be prosecuted too.
On the particular problem of the ACC, the Revenue could call in the bank's principal shareholder, the Minister for Finance. These measures, he suggested, would lead to significant gains in the yield from DIRT.
Here then was a clear and explicit plan, drawn up by a respected senior official, which did not necessarily involve any extension of the Revenue's powers.
What happened to it?
It went, in the first instance, to the chief inspector and deputy secretary of Revenue, Christopher Clayton. As he explained to Jim Mitchell on Tuesday, "At the time I was not in a position to examine it. It obviously was a major work and it raised very serious and delicate issues. I couldn't, in the circumstances then prevailing, give it the time and the attention that would have been appropriate for such a paper."
He also suggested that there were political considerations: there had been ongoing correspondence with the Department of Finance which indicated that the Minister didn't want any changes in the position about access to non-resident accounts.
Sean Moriarty's paper was, however, read by the board of the Revenue Commissioners. This body, the ultimate authority in the field of tax collection, was, as Pat Rabbitte put it to its then chairman, Cathal Mac Domhnaill, on Thursday, not exactly a model of modern management practice: there were no regular board meetings, there was no agenda, there are no minutes.
Mr Mac Domhnaill and his colleague Dermot Quigley rejected Sean Moriarty's proposals. In his evidence, Mr Mac Domhnaill suggested that the main reason for the rejection was that the Moriarty paper supported a tax amnesty, to which the Revenue board was implacably opposed.
This was, no doubt, his sincere conviction, and it seems a laudable one. The difficulty for the PAC, however, is that Sean Moriarty's paper does not in fact argue for a tax amnesty. It merely states, more or less as an aside, that the direct approach to the financial institutions at a high level should take place in tandem with an amnesty announcement if any such amnesty were to incorporate bogus non-resident accounts.
None of Mr Moriarty's proposals was in any way dependent on the granting of an amnesty. That the chairman of the Revenue rejected them under such a misapprehension may seem extraordinary. But in the light of the rest of the week's evidence, it was just another enigma in a maze of mysteries.