Brussels released a letter which the EU's competition commissioner sent to the Irish government in June formally giving its reasons for opening an investigation into Dublin's arrangements with Apple.
The letter from Joaquin Almunia said the Irish government's treatment of Apple "appears to constitute state aid" and therefore breaches the 28-member EU's rules on a free internal market.
"The Commission is of the opinion that through those (tax) rulings the Irish authorities confer an advantage on Apple," the letter said.
"That advantage is obtained every year and on-going, when the annual tax liability is agreed upon by the tax authorities in view of that ruling."
It asked Ireland to provide further information about the deals, which have benefitted Apple since 1991.
Ireland said on Monday that it was confident there was no breach of state aid rules.
If found at fault, however, Apple would be required to repay potentially billions in restitution.