It was to be one of his final acts of duty as deputy governor of the bank.

O'Brien was carrying a letter of resignation with him and he told colleagues of his plans to stand down.

The following day, O'Brien sat in at the full board meeting until the business of his resignation was dealt with, after which he left the room for the last time.

There was no real sense of shock at the bank. While the entrepreneurial flair that O'Brien brought to the table will be missed, his was purely a nonexecutive role and life will continue exactly as normal at one of the country's most profitable institutions.

There was also some understanding of the decision, given the huge demands that his rapidly growing Caribbean mobile phone company Digicel is placing on O'Brien's time.

While O'Brien had been a good attender of board and committee meetings, it had been noted that several times when discussions were going on about a trading statement or a set of results, the businessman was being patched into meetings while 30,000 feet above the ground in his private Gulfstream jet.

But there was some surprise at the suddenness of the decision. There had been no musing or taking of soundings before O'Brien delivered the news. And, after all, it was only a year since he had taken up the highly prestigious post as deputy governor.

On one level, the rationale for the decision to quit Bank of Ireland, and other non-executive directorships O'Brien held, is obvious.

"Bank of Ireland is worth 10bn and Digicel is worth around $2.5bn.

Denis has a few shares in one and owns 90% of the other, " one close observer commented, indicating that O'Brien had rather logically decided where his priorities lay.

By any standards, Digicel is a stunning success story that is on the verge of catapulting O'Brien from the status of hugely wealthy into an elite club of the mega-rich.

Rather than sitting back and enjoying the hundreds of millions he earned from the sale of his Esat Telecom business, O'Brien has, in just five years, built a company from scratch that sources say will be valued "north of $2bn" when it is floated on the New York stock exchange in December or January.

And that's only the Caribbean operations. It doesn't include Digicel Pacific . . . which plans to build a Pan South Pacific GSM network and already has licences in Samoa, Fiji and Papua New Guinea . . . or O'Brien's plans to launch a 'virtual' telecoms business in the US market.

This success has been achieved, sources say, by O'Brien "leading from the front". That means spending two weeks a month in the Caribbean. The demands on his time will only increase with the pending flotation. Throw in his Communicorp radio interests across Europe and reports of a new address in Malta . . . the island that charges no tax on assets or income not brought into the jurisdiction . . . and you could argue that the only surprise is that O'Brien took up the Bank of Ireland position in the first place.

But the fact is that he did take it up, and not that long ago either.

Inevitably, the suddenness of his decision to depart has prompted conspiracy theories.

The Moriarty tribunal is still examining the circumstances under which O'Brien and Esat Digifone was awarded the second mobile phone licence . . .

the most lucrative commercial licence ever issued by the state . . . more than a decade ago.

It is important to stress that no evidence of any wrongdoing has ever been uncovered about O'Brien. However, the investigation has thrown up some extremely interesting revelations, not least of which was that O'Brien had once said to the then Digifone chief executive Barry Maloney that he, O'Brien, had made, or tried to make, a payment of £100,000 to former communications minister Michael Lowry.

O'Brien has dismissed the comment as blather and told the tribunal he did not make any payment.

However, it also emerged that, when Lowry was minister, money went from the Dublin bank account of a Dutch company controlled by O'Brien to a bank account in the Isle of Man in the name of O'Brien's accountant, and then on to the Jersey bank account of David Austin, a Smurfit executive and friend of both O'Brien and Lowry. That money was in turn routed back to the Isle of Man to an account set up in Lowry's name.

O'Brien has said the money was for a house in Spain he bought from Austin, while Lowry has said the money was a loan he got from Austin.

The tribunal's finding on this is eagerly awaited, not least by the losing consortia in the 1995 mobile licence competition.

Last week, the rumour mill was in full swing suggesting that perhaps the tribunal had written to O'Brien with an outline of its indicative findings . . .

part of the process to allow people affected by the tribunal's rulings time to respond before its final report . . . and that this lay behind O'Brien's decision to resign from Bank of Ireland.

It is understood, however, that while the tribunal has issued outline indicative findings to up to 200 people in foot of its investigations into Charles Haughey, it has not done so in relation to its work on Lowry.

Well placed sources say that it could be some time before the tribunal is in a position to do so, as it has yet to deal with a couple of crucial and time-consuming issues . . . including its inquiries into the purchase of English soccer club Doncaster Rovers and whether Lowry was involved in the deal . . . now scheduled for November.

"There is no question of any report any time soon, " one well-placed source claimed, adding that it would be well into next year before it could be expected.

But the word is that the Moriarty has taken a particularly tough line in relation to the Haughey findings, with several people, in the words of one source, "getting a terrible doing".

Nobody was expecting this "savage" approach and it has "scared everyone", the source added.

It is impossible to know if this fallout in any way influenced O'Brien's thinking. But what is certain is that suggestions of a link between the tribunal and his decision last week to quit the bank will infuriate O'Brien.

While friends say he has been getting on with life, the tribunal has been hanging over him for more than five years now and in many ways has overshadowed his achievements in the business world.

O'Brien has spent millions on advisers as a result of the long-running probe . . . including the cost of a number of legal challenges to the tribunal . . . and it is impossible not to sympathise with the view that the length of the tribunal is taking has been unfair on the likes of O'Brien and Lowry.

It does, at last, seem that the end game is approaching. When O'Brien was appointed deputy governor last year, it was regarded by many as final proof that what happened up at Dublin Castle no longer mattered in the "real world".

But now O'Brien's resignation from Bank of Ireland . . . no matter how unconnected it may be to Moriarty . . . has reminded us all that the tribunals haven't gone away. Not that Denis O'Brien would have needed reminding of that.