Ryanair has said it is assisting German tax authorities with an inquiry into a number of contractor pilots working for the airline.
It is understood a number of pilots were questioned by authorities in Germany, and there are reports that the homes of some pilots were raided.
In a statement Ryanair said the company itself was not the subject of any tax investigation.
The Ryanair Pilot Group, which represents pilots from a number of countries, said it had received reports today of pilots in several bases in Germany being interrogated about their financial affairs, and in at least one base it said it had a report of pilots' homes being raided.
It said it was working with the pilots to establish why the authorities there had taken such actions.
The group said it was concerned about the welfare of crew members and called on the aviation authority to ensure that pilots who are due to fly in the coming days are in a fit condition to do so.
Ryanair said it requires all its pilots, both directly employed and contractor, to be fully tax compliant at all times and the company will provide the German authorities with any further assistance.
In a statement to RTÉ News, the airline added: “The so-called Ryanair Pilot Group is a collection of competitor airline pilots from KLM, Air France and Aer Lingus.
"It does not speak for or represent any Ryanair pilots. The only thing this so-called group has in common is that none of these pilots fly for Ryanair.”