The State's appeal in the Apple tax ruling will be heard by the EU General Court in Luxembourg later today.
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.
Explaining the decision in August 2016, the EU's Competition Commissioner Margrethe Vestager said: "The Commission's investigation concluded that Ireland granted illegal tax benefits to Apple, which enabled it to pay substantially less tax than other businesses over many years.
"In fact, this selective treatment allowed Apple to pay an effective corporate tax rate of 1 per cent on its European profits in 2003 down to 0.005 per cent in 2014."
The Commission ordered the tech giant to pay over €13 billion in back taxes and interest to Ireland to make up for the shortfall.
As a result of the ruling, Ireland was forced to collect the billions of euro from Apple.
The money is being held in an escrow account until the appeals process has concluded, as Apple and the Irish State have challenged the ruling.
Over the course of today and tomorrow, the State will attempt to have the original ruling reversed.
Lawyers for Ireland will argue the State does not treat one company different to another.
They're alleging that the Commission "made manifest errors" in both its state aid assessment and understanding of Irish tax law.
Apple itself has also taken a case against the Commission.
The EU General Court (EGC) in Luxembourg is the bloc's second highest court.
Luxembourg is supporting Ireland in the appeal, while Poland is supporting the Commission.