The Finance Department has confirmed that Ryanair has lodged a legal challenge to one of the country's tax laws.
The tax measure, introduced in the 2011 Finance Act, provides for income earned by flight crew members who work on Irish-registered and managed aircraft that operate in international traffic to be taxed.
"As this matter is the subject of ongoing legal proceedings, we will not be commenting further," a Finance Department spokesman said in a statement.
Ryanair CEO Michael O'Leary told the airline's AGM in September that it would lobby the Government to change tax laws to allow them to grant union demands that workers would be given local contracts governed by the law where they are based, rather than by Irish law as at present.
A spokesman for the Revenue Commissioner said it could not comment on the matter due to taxpayer confidentiality.