AIB's senior counsel, Mr Dermot Gleeson, has managed to break new ground for the bank which could constitute a significant mitigating factor in any settlement it may have to make with the Revenue Commissioners. And judging by the growing strength of the Revenue's argument, a settlement seems to be on the cards.
In a last-ditch attempt to establish proof that it negotiated an agreement with the Revenue in 1991 which absolved it of paying any DIRT arrears, Mr Gleeson subjected three senior Revenue officials to rigorous questioning on their knowledge of and involvement in these discussions with the bank.
The Revenue officials stood up well to the former Attorney General's close scrutiny of their notes, which record what took place at a meeting with AIB in February 1991, with each one strongly asserting there never was an agreement to write off tax.
The breakthrough for the bank came when Mr Gleeson highlighted the degree of complexity which is involved in establishing just who is deemed to be resident in the Republic at any time for tax purposes.
This goes to the heart of the bogus non-resident accounts issue, and he was seeking to show that it would have been very difficult for bank staff to determine the authenticity of an individual's tax status.
Mr Gleeson pointed out the absence of legislation which defined residency until 1994 and the fact that some individuals can be legally deemed to be non-resident for tax purposes even though they spend long periods of time in the Republic.
He suggested: "A non-resident can keep his non-resident status and show up 183 days of the year. He could go to Mass every Sunday and shake hands with the bank manager as he comes out. He could call to the bank on Monday morning. That's 104 days."
The Revenue senior inspector of taxes, Mr Tony Mac Carthaigh, agreed that account-holders could be widely seen in the locality and still be legally entitled to be treated as a non-resident for tax purposes.
But he stressed that based on his knowledge of the scale of the abuse of non-resident deposit accounts in the 1980s and early 1990s, the problem as far as the Revenue was concerned was that bank staff were opening these accounts for the likes of local shopkeepers and neighbours as a means to evade tax.
AIB has made the point sufficiently strongly to cast considerable doubt on the scale of culpability that could be attached to its officials in opening bogus non-resident accounts.
Indeed other financial institutions will also be quick to cling to this argument if it is accepted as a mitigating factor when the Revenue makes an estimate for unpaid DIRT.
The bank also established that the Revenue does have the power under its care-and-management provisions to "fail to collect taxes" in certain situations, suggesting it was always in its gift to make an agreement such as AIB claims to have secured.
But despite much effort, Mr Gleeson failed to prove that the Revenue ever made such an agreement.
The Revenue, through its senior counsel, Mr Eoghan Fitzsimons, has made steady progress in defending its position. In his examination of AIB's former head of compliance, Ms Deirdre Fullen, Mr Fitzsimons has found a number of inconsistencies in her notes of telephone conversations and meetings with the Revenue in 1991.
Crucially, he established that AIB could not comprehensively state the date it believes it concluded its so-called amnesty with the Revenue.
Ms Fullen told the committee she believed it was probably in December 1991 when in a phone call with Mr Mac Carthaigh, he indicated that AIB had held up its part of the deal in tackling bogus non-resident accounts and that it was time for the Revenue to honour its side of the bargain.
Mr Fitzsimons later established the bank's auditors, PricewaterhouseCoopers, had accepted the deal had been concluded much earlier than that, signing off its annual results in May 1991 without making any provision for unpaid DIRT.
Today the Revenue will continue its examination of two other AIB officials who were in its tax department in 1991, Mr Jimmy O'Mahony and Dr Donal de Buitleir.
As the curtain begins to fall on the DIRT inquiry, the question of just how much AIB could owe to the Revenue still has to be settled.