The Supreme Court has begun hearing an appeal by the former chief executive of Irish Life and Permanent against his conviction for his part in a multi-billion euro conspiracy in 2008, to deceive people about the true state of Anglo Irish Bank.

Denis Casey claims he should have been allowed to use the role of the Financial Regulator, Central Bank and the Department of Finance as a defence in his trial.

Casey was found guilty of a conspiracy to defraud and sentenced to two years and nine months in jail in July 2016.

The jury found he conspired with Anglo Irish Bank's John Bowe and Willie McAteer to engage in circular, non commercial transactions between Anglo and the IL&P group, aimed at fooling people into thinking Anglo had received an extra €7.2 billion in customer deposits.

Anglo's former boss, David Drumm, was convicted of the same offence last year.

Senior Counsel Michael O'Higgins said Casey authorised a series of transactions with Anglo Irish Bank in March, June and September 2008. 

He said these transactions were in compliance with the "green jersey agenda" which he said was drawn up by the Financial Regulator. 

Mr O'Higgins said this meant that Irish Banks were to help each other out at a time of unprecedented global financial turmoil, particularly when year end and half year figures were due to be published.

Mr O'Higgins said Casey was interviewed by gardaí on several occasions and a strong pillar of his defence was that the transactions would be scrutinised by a number of "gatekeepers" including the Financial Regulator and Anglo's auditors, prior to the results being published to the market.

And he said if there was anything remiss in what had been done, then the accounts would not have been published in the way they were.

Mr O'Higgins told the court that between 30 September 2008, when the transactions at the centre of the case were completed and 3 December 2008, when Anglo's interim results were published, the Financial Regulator was all over every single aspect of the transactions and did not raise any quibble or query about their propriety.

He said that the State contended that the cut off point for any attempt to rely on the "green jersey agenda" as a defence was 30 September.

But he said even if that were the case, the court was entitled to look at his client's conduct after that time.

The prosecution says there is no evidence that the Financial Regulator and Central Bank were aware of or supported the transactions.  The judge at Casey's trial found such an argument could be used only in mitigation - to reduce a sentence - and not as a defence.

The Supreme Court has to decide whether or not the defence of officially induced error, is available here and whether it was open to Denis Casey.

Lawyers for the prosecution, Senior Counsel Paul O'Higgins, said the green jersey agenda was to the effect that the banks should support each other in relation to liquidity.

Mr O'Higgins said it was a perfectly legal strategy and could not be seen as having any role in precipitating this offence. 

He said the defence of officially induced error relies on a representation about the legality of the alleged offence being made in advance of the offence being carried out.

He said there was no such representation here and the defence was not available to Casey.

Five judges are hearing the case which is expected to last all day.  They are expected to reserve their decision.