AIB is unable at this stage to estimate its liability for DIRT tax arrears between 1986 and 1991, although it has described the figure of £100 million as "off the wall".
Chief executive, Mr Tom Mulcahy, said the bank had been unable to arrive at a figure despite a request from the Revenue Commissioners.
He said the bank "is working on one" at present and will inform the Revenue as soon as possible. If a large tax liability were discovered, it was likely that it would be met from the bank's own resources and not debited to customer accounts.
When asked did AIB "quantify" its DIRT liability in 1991, Mr Mulcahy said the bank did not "as it was not asked to, then or later".
Mr Mulcahy said the bank understood that the Revenue did not look for an estimate of the liability arising from the arrears because what AIB agreed with them "was a forward-looking arrangement and there was no reason to get an estimate".
The current head of AIB group taxation, Mr Philip Brennan, said if the bank had a liability of £100 million "it would be beyond comprehension for us not to include such a figure in the bank's accounts".
He said it was very difficult in the 1980s and early 1990s to find out if someone was a genuine non-resident. He said challenging somebody about their personal financial affairs could mean "you ended up getting sued".
When Mr Sean Doherty (FF) asked was it AIB's understanding that if arrears arose later from the pre-1991 period the bank would not be liable for them. "That is our core contention," Mr Mulcahy replied.
Mr Brennan said the "passage of time" made it difficult to arrive at an estimate for DIRT liability. He said many records would now be lost and there were dangers in "doing a back-of-an-envelope calculation".
The chairman of the Dail Committee of Public Accounts, Mr Jim Mitchell (FG), said: "I find it hard to believe that a company with your resources cannot do some kind of calculation of what the likely liability would be." He added that it was "surprising that AIB are prepared to rubbish Mr Spollen's figures and describe them as off-the-cuff, but are not able to produce their own figures".
Mr Mulcahy said there were four variables making it difficult to reach a figure on DIRT liability. He said they were the changing volumes of non-resident accounts over the period, changes in interest and tax rates and changes in the certification of the individual accounts.
He did acknowledge, however, that "there were problems and there is no doubt there were arrears".
He added that Mr Spollen's figures could not be used as a starting point for calculating any liability as they were "not reliable".
He said the bank "has not made one ounce of profit out of this" and "would rather that the accounts were DIRT-able as it would help us sleep better at night".
After the Revenue cleaned up the system dealing with non-resident accounts in 1991, Mr Mulcahy said the bank lost some customers who moved their money elsewhere. "I couldn't put a figure on it, the cases were numerous, but not gigantic," he said.
