Irish and Spanish aviation authorities on Tuesday agreed to jointly investigate the diversion to Madrid of a Ryanair flight from Paris to Tenerife following a technical issue.
The decision came at a meeting of air safety experts in Dublin which followed a series of comments by Spanish authorities about incidents in Spanish airspace involving Europe's largest budget airline.
Spanish authorities are investigating emergency landings by three Ryanair planes in Valencia on July 26 after they approached their minimum required fuel.
Ryanair has said the July landings were forced by bad weather and that it complies fully with EU safety procedures.
Spanish authorities are also looking into an incident in which a Ryanair flight diverted and landed at Barcelona's El Prat airport after a possible engine fault was detected.
The investigation announced on Tuesday follows a diversion on Sunday. Ryanair said in a statement the flight from Paris to Tenerife was diverted to Madrid.
Ryanair has accused the Spanish aviation authorities of falsifying information on incidents involving its planes, an accusation Spanish officials have rejected.
"The Irish authorities gave an assurance of the Irish Aviation Authority's rigorous oversight of Ryanair's operations and on their satisfaction with Ryanair's safety standards which are on a par with the safest airlines in Europe," the Irish department of transport said in a statement after the meeting.
Ryanair, which last year carried over 30 million passengers in Spain, is the country's largest passenger carrier.
Spain's Public Works minister Ana Pastor, whose ministry runs aviation safety, has called for tighter safety regimes at low-cost airlines following a series of media reports about emergency incidents.
Ryanair has rejected claims by the Irish pilots' union (IALPA) that it pressures flight crew to carry the minimum amount of fuel required under European regulations.