The State's appeal of the Apple ruling will be heard by Europe's second highest court in Luxembourg later today in what is being described as the world's biggest tax case.
A two year investigation by the European Commission concluded in 2016 that the Irish state allowed Apple to pay far less tax than other companies.
It ordered the tech giant to pay over €13bn in back taxes and interest to Ireland to make up for the shortfall.
However Apple, along with the Irish State have always fought against this.
The latter will argue it doesn't treat one company different to another, when it goes before the General Court later.
Meanwhile, Ireland has since been forced to take the billions of euro from Apple.
The money is being held in an escrow account until the appeals process has ended.
Over the course of today and tomorrow, both the State and Apple will aim to have the original ruling reversed.
The court will then decide whether the Commission was right to place the record tax bill on the tech giant.