Aer Lingus's plans to transfer Shannon Airport's London Heathrow service to Belfast International may be blocked.
Michael O'Leary says he will attempt to block the move
Ryanair has a 25% stake in Aer Lingus, and its chief executive wants an extraordinary general meeting where it could use its vote to oppose the plans.
Michael O'Leary said both Shannon and Belfast could still have London flights if Aer Lingus transferred its Dublin- Gatwick service north to Belfast.
Aer Lingus pilots are to strike over plans to pay Belfast staff less.
All routes will be affected, with 500 pilots set to refuse to fly next Tuesday and Wednesday.
The airline last week announced 100 new jobs at Belfast International Airport, but employees will not be paid the same rates as staff in the Irish Republic.
A pilots' union spokesman said they had to make a stand, but the Irish national carrier is refusing to back down and accused them of brinkmanship.
Michael Landers of the union Impact said the strike action was important.
"If we didn't take up that line, I think the inevitable outcome would be that Aer Lingus would use the pilots at the new base to eventually drag down pay and terms and conditions throughout the rest of the company as well," he said.
But Aer Lingus Chief Executive Dermot Mannion told the BBC's Good Morning Ulster the company had already checked the legality of its policy.
"We have been through all the available machinery of the state here, (and) we have had an arbitration process specific to the pilots sponsored by the Irish labour courts," he said.
"Both of those processes agree - it is appropriate in new bases outside the Irish Republic to recruit on local terms and conditions."
It is understood Aer Lingus is seeking to recruit up to 30 pilots for its Belfast operation, as part of a £100m investment.
The move has sparked protests in the Irish Republic, with fears of job losses at Shannon.
More than 50% of Aer Lingus is owned by Ryanair, the Irish State and the unions.
Ryanair has promised three additional flights from Shannon to various London airports if Aer Lingus closes the Heathrow route.
Ryanair owns a 25% stake in Aer Lingus
Mr O'Leary told a news conference: "If Aer Lingus cancels its Shannon-Heathrow flights, then it is inevitable that the Shannon transatlantic flights will also transfer to Belfast Aldergrove in the not too distant future.
"The government and Aer Lingus's determination to invest in Belfast should not come at Shannon's expense."
Ryanair begins flights from Belfast City Airport to three UK destinations in October.