Ryanair announced on Friday that director Sean Coyle had quit to become chief financial officer at rival Aer Lingus, in which Ryanair holds a 29 percent stake after a failed takeover bid.
Former Irish state carrier Aer Lingus said in a separate statement that Coyle, Ryanair's director of scheduled revenue, would join the group on August 11.
"Challenging times bring unique opportunities and we are delighted that Sean is joining us," Aer Lingus Chairman John Sharman said.
Coyle, 35, an accountant, worked at KPMG before joining Ryanair in 1998.
"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," Ryanair Chief Executive Michael O'Leary said.
The European Commission last year blocked Ryanair's bid for Aer Lingus on competition grounds but subsequently also rejected a request from Aer Lingus that it force Ryanair to reduce its stake.
A European court this year rejected an attempt by Aer Lingus to issue an interim order blocking what it called interference by Ryanair.
Aer Lingus warned earlier this month it would at best break even this year due to a slowing world economy, rising fuel costs and a strong euro.
Ryanair said early in June that it would break even in its current financial year if oil prices remained at USD$130 a barrel. On Friday oil hit a new record high near USD$142.
