The Air Travel Tax was applied from 2009 until 2014
Ryanair say they are prepared to take the Irish Government to court, after a ruling found they are partly liable to pay millions in a legal row over the former travel tax.
The European Court of Justice (ECJ) previously ruled that the travel tax was discriminatory, because passengers from Dublin paid a lower rate than those from other airports.
But it also told the State to recoup the difference from the airlines.
This leaves Ryanair and Aer Lingus to pay €8 for every passenger who benefitted from the lower rate.
The court has now rejected an appeal from the European Commission.
They have to pay extra travel taxes on behalf of every passenger they carried from Dublin while the tax was in operation.
The Air Travel Tax (ATT) was applied to passenger departures from March 30th 2009 until March 31st 2014.
Up to February 2011, a rate of €2 was applied where the flight was to a destination no more than 300 kilometres from Dublin Airport.
This covered all flights within Ireland and flights to the UK. While a rate of €10 applied to flights to any other destination.
In a statement, Ryanair say: "We have been expecting this judgment on Ireland's illegal Air Travel Tax which now requires Ryanair to pay some €12m to the Irish Government on behalf of passengers who paid the lower €2 tax rate on shorter routes, despite the fact that we did not collect the €10 tax from these passengers.
"This ruling now clears the way for Ryanair (and other airlines) to pursue our High Court action against the Irish Government to recover the €88m of damages we suffered as a result of being forced to pay this illegal tax."