In doing so, Ryanair was able to sidestep labor regulations and social security taxes in the dozens of countries in which it operates. Its labor costs were among the lowest within its European cohorts. Other low-cost rivals followed its lead.

While pilots have pushed to unionize for years, Ryanair was able to hold off their efforts. But a scheduling disaster in recent months put Ryanair on the defensive.

After the airline messed up its fall vacation schedule for pilots, it scrambled to find replacements, saying instead that it might cut a week of vacation time. Employees, many of them independent contractors, pushed back. Ryanair ultimately had to cancel more than 20,000 flights, affecting hundreds of thousands of passengers in all.

Pilots galvanized to set up their own groups across Europe for negotiating with the company. They demanded collective bargaining and more secure contracts.

Ryanair resisted for months. It was concerned that rivals were trying to leverage the flight cancellations to force it to negotiate with unions and, potentially, push its labor costs higher. Ryanair’s chief executive, Michael O’Leary, argued that airline unions and competitors were trying to “demean and disparage our collective success.”

As the busy holiday travel season approached, pilots ultimately threatened to strike. Ryanair had to relent.

It invited unions in Britain, Germany, Ireland, Italy, Portugal and Spain to talks.

“Christmas flights are very important to our customers, and we wish to remove any worry or concern that they may be disrupted by pilot industrial action next week,” Mr. O’Leary said in a statement.