Ryanair German pilots decided on Thursday to walk out. They will join striking pilots in the Netherlands and Belgium.
Cabin crews in Belgium, Germany, Italy, the Netherlands, Portugal and Spain will also go on strike in a row over contracts and conditions.
Unions want staff to be given contracts in the countries where they live, rather than under Irish law. EU social affairs commissioner Marianne Thyssen told Mr O'Leary at a meeting in Brussels on Wednesday that EU rules on employment of air crews were based on where workers left in the morning and returned in the evening - and not where aircraft were registered.
Joost Van Doesburg, of the VNV union, the Dutch pilots union, said his members also wanted pensions in line with Dutch standards, and firmer guarantees on sick pay.