Details of how multinational companies' tiny tax bills are calculated are to be revealed by the State for the first time.
The information, concerning the tax affairs of US companies such as Apple and Google, is to be handed to European authorities investigating whether "sweetheart" deals are being offered to businesses operating here.
But tax details – which will be provided by the Revenue Commissioners directly to the European investigators – will not be made public.
The Government and the companies involved will be very anxious that this commercially sensitive information does not get into the public domain.
Irish authorities have always insisted that there are no special tax deals for companies. Under Irish law, all businesses are supposed to be subject to the same laws and tax rates.
But critics say that secret "tax rulings" by Revenue officials for individual companies can be almost as valuable to a business.
It is the details of these rulings that European Commission officials will now be able to consider.
This is the first time information about how Ireland taxes big corporations has ever been shared outside of the Revenue Commissioners and the companies themselves.
If any evidence of abuse emerges from the initial probe, a full investigation will be launched by the European Commission.
Tax rulings are so confidential that even the Department of Finance is never given details by Revenue of individual cases.
The new probe has nothing to do with Ireland's corporate tax rate of 12.5pc.
European authorities are instead looking at whether companies or even sectors are getting an unfair advantage because of the way their tax bills have been worked out.
In the tax rulings, the Revenue Commissioners tell companies what income they will be taxed on in Ireland, and the income not liable for tax here.
The confidential rulings will help explain why giants such as Apple and Google pay little or no corporation tax here, even though their Irish-based subsidiaries have turnovers running to billions of euro a year.
The news that the tax data is to be shared comes after it was revealed that the European Competition Commission had launched an informal information-gathering probe to find out how taxes are being levied in Ireland, the Netherlands and Luxembourg.
Ireland hit world headlines for all the wrong reasons in May when an investigation by the US Senate found that computer giant Apple was paying only 2pc of its income in tax, thanks in large part to Ireland's assessment of the tax bills of Apple subsidiaries based here.
One Apple subsidiary incorporated in Ireland paid no tax anywhere on $30bn of revenue between 2009 and 2012, the US senators claimed.
Apple told them that it had a special deal with Ireland, sparking outrage in Washington.
The Government says that is incorrect, but Apple, like all companies, does have a "tax ruling" setting out what income it is and is not to be taxed on in Ireland.
Taoiseach Enda Kenny insisted the State is working with international partners "to bring about a transparent and accountable system in respect of tax".
There was support for Ireland from the head of the Organisation for Economic Security and Development (OECD), a club that helps cross-border coordination of tax policies.
But the days when multi-nationals could avoid tax altogether are over, and that is not just an Irish issue, he said.
"It is not about low taxes or high taxes. The question is how do you make sure that companies that operate in a particular market actually pay taxes in that jurisdiction," he added.
A government source said: "We'll fight the accusation that Ireland does 'tax deals' with any company. We have a transparent tax regime."
By Donal O'Donovan, Michael Brennan and Colm Kelpie