Ryanair's director of scheduled revenue, Sean Coyle, has resigned to take up the position of chief financial officer with Aer Lingus.
At Ryanair since 1998, Sean Coyle was in charge of route selection and profitability, network strategy and capacity allocation.
He will join Aer Lingus on August 11 and subsequently take up a position on the airline's board.
Ryanair's chief executive Michael O'Leary wished Mr Coyle every success in his new role, saying: 'We are pleased to see Aer Lingus recruit some decent management and hope that Sean will introduce some of the changes we have been calling for in Aer Lingus.'
Aer Lingus chairman John Sharman said Mr Coyle has proven experience and that he will significantly strengthen its management team.
Ryanair is a 29% shareholder in Aer Lingus.
