Ryanair has initiated High Court proceedings against its departing chief operating officer, Peter Bellew.
The airline announced in a memo to staff last month that Mr Bellew was leaving at the end of the year, after almost two years back at the company.
Just a week later it emerged that he is set to join rival Easyjet as chief operating officer.
The nature of the legal proceedings being brought against Mr Bellew by Ryanair have not been made public.
But according to the Courts Service website, Ryanair initiated the action on Tuesday.
The company had no comment to make about the matter when contacted, while Mr Bellew also declined to discuss the issue when contacted by RTÉ News.
The development was first reported by the website Fora.
After stepping down as chief executive of Malaysia Airlines, Mr Bellew joined Ryanair in December 2017 just as the Irish low-cost giant struggled with a pilot shortage that forced it to cancel around 20,000 flights.
Mr Bellew had been director of flight operations at Ryanair prior to leaving it in 2014 and joining Malaysia Airlines as chief operations officer in September 2015. He later became chief executive in July 2016.
In a call with analysts last week, following publication of the airline's financial results, Ryanair chief executive Michael O'Leary was asked about Mr Bellew's departure to rival Easyjet.
"All of the senior management in Ryanair have pretty long termination notice periods in their contracts," he said in the call.
"In Peter's case, that's currently six months. And thereafter, they have a pretty extensive non-compete agreement, which arise from the share option schemes that they have agreed and received over a period of time.
"So I would not expect any senior manager in Ryanair to be moving to a competitive airline for a reasonably long period of time."
He added that the airline is in discussions with Mr Bellew about his termination period, but that it would not be moving on its contractual requirements.