Fórsa has signed a deal with Ryanair making it the sole negotiating union for the airline's directly employed cabin crew in Ireland.
The deal was signed on Wednesday by Fórsa officials, cabin crew representatives and Ryanair management.
Fórsa has called it a "new, and very significant, milestone" in collective bargaining at the company, which only agreed to recognise union last December.
Fórsa official Ashley Connolly insisted the union will not be intimidated by Ryanair's tough approach to industrial relations.
She said: "I think that Fórsa has proven itself over the years to be equally as tough when required and when needed.
"I think it's important for the first time that Ryanair cabin crew members themselves have a voice - they will be to the forefront of any discussions I have."
Around 100 Irish-based pilots with Ryanair are currently being balloted by Fórsa on a deal reached to end the dispute between the union and airline over seniority issues and base transfers.
The dispute resulted in five days of strikes and the cancellation of dozens of Irish flights, with the union now calling on its members to accept the agreement reached last week.
Ryanair said it looks forward to working with Fórsa and the elected Ryanair council.
The airline's chief people officer, Eddie Wilson, said: "We are pleased to sign this cabin crew recognition agreement with Forsa in Ireland.
"We look forward to working closely with both Forsa and their company council to address issues of concern to our directly employed Irish based cabin crew.
"This is a further sign of the progress Ryanair is making with trade unions since our December 2017 decision to recognise unions, with over 65% of our cabin crew now covered by recognition agreements and we hope to sign more agreements in the coming weeks."
Additional reporting: Jack Quann