For many Ryanair employees, the strikes roiling the airline this summer are not about the money.

The key issue uniting Ryanair employees in their anger against their employer is contracts. Employees in many countries want employment contracts governed by the laws of the nations where they are based, not Irish law as is currently the case (except for staff based in the U.K.).

While unions call it a "modest" demand, Ryanair Chief Executive Michael O'Leary has flatly refused to consider the idea. But his stance is coming under fire as pilots from Germany, Sweden, Ireland, the Netherlands and Belgium go on strike Friday, canceling about 500 flights and affecting tens of thousands of passengers.

The reason O'Leary is holding firm is that the widespread use of Irish labor contracts for its 14,500 workers is a key to keeping Ryanair's costs down and to being able to quickly redeploy employees.

"If local contracts were imposed it could impact on costs, productivity and complexity of the business," the carrier said in its latest annual report. Ryanair has been willing to give pilots a big boost in pay and broke with precedent in agreeing to recognize unions, but it's refusing to budge on contracts.

'Flag of convenience'

Irish employment law is more liberal than in many other European jurisdictions, Dutch Airline Pilots' Association spokesperson Joost van Doesburg explained. "Irish law is produced for Irish companies to provide more flexibility. Dutch law protects Dutch employees."

Van Doesburg said that Irish law makes it easier to fire workers and harder to sue employers. He argued that since Ryanair has employees and bases all over Europe, it isn't an Irish airline, but a "European airline." He alleged that Ryanair is using Irish law as a way to skimp on providing for its workers, calling Irish law just a "flag of convenience."

The Spanish pilots' union SEPLA filed a lawsuit against Ryanair in Spain to force the airline to move its local employees to Spanish contracts. It provided POLITICO with a document illustrating the differences between Irish law and Spanish law. It highlighted that Spanish law mandates paid leave for weddings, limits on working hours, and reduced hours in case of emergencies. Irish law does not.

But O'Leary doesn't believe the unions have an argument. "We're an Irish airline. We're not a Spanish airline," he said at a June briefing in Brussels. All of Ryanair's 400-odd planes are registered in Ireland (O'Leary himself is Irish). Asked about Ryanair's Spanish pilots suing the airline on the issue, O'Leary said: "Sue. Off you go, get on with it," adding: "You'll lose."

Irish contracts “are perfectly valid under the Treaty of Rome,” O’Leary said, referring to one of the EU’s founding treaties laying out the blueprint for the single market.

He cited an earlier petition to change Ryanair's contracts that was turned down by the Valencia Court of Appeals. However an appeals court in Den Bosch, the Netherlands, ruled last month that Dutch law must apply to Ryanair's employees based there.

The European Court of Justice weighed in on the issue of where Ryanair employees can take their grievances last year. It ruled that employees have the option of "bringing proceedings before the courts of the place where they perform the essential part," of their duties, but not exclusively. The judgment was seen as a win by both Ryanair and the unions, which suggests that the issue of jurisdiction still isn't resolved.

It's just business

The registry of Ryanair's aircraft notwithstanding, the reason why the carrier is insistent on Irish contracts is because it saves the airline money, unions say. Fewer protections and a cheap, flexible workforce is part of the company's formula to keep ticket prices low and planes in the air.

"The business model won't work outside Ireland," said Dirk Polloczek, president of the European Cockpit Association, which represents the interests of national pilot unions in Brussels. "As soon as you have to pay social taxes in other countries, that business model is gone."

"We're the ultimate opportunistic airline" — Michael O'Leary, Ryanair chief

Ryanair closed its only French base in Marseille in 2011 after a French court ruled that its employees had to be subject to French labor laws. But the airline may reverse its decision as travel to France becomes more lucrative, a step made easier by its policy change on unions. The airline is currently negotiating with French pilots to re-establish two bases in France given that it forecasts its capacity to double in the next four years to 20 million passengers. Reuters reported that those bases will have employees based on French contracts.

"We're the ultimate opportunistic airline," O'Leary said in Vienna earlier this month.

This article has been updated with German pilots striking.