Mr Bellew, who now runs Malaysia Airlines, will take on responsibility for all of Ryanair's flight operations, ground operations and engineering.
Mr O'Leary said: "Peter has an unrivalled knowledge of our business model and how we maintain controlled growth, with industry leading punctuality, for the benefit of our customers and our people".
Malaysia Airlines, which under Bellew's stewardship just announced a plan to buy Boeing aircraft worth 10 billion US dollars during Malaysian Prime Minister Najib Razak's visit to the United States, maintained that the turnaround of the group remains on track and on schedule, as per the MAS Recovery Plan.
Bellew's predecessor, Christoph Mueller quit his three-year contract in April 2016, after having been in the job for just over a year.
In response to Bellew's resignation, Malaysia Airlines said in a statement that its board of directors will meet and an announcement will be made in due course.
Bellew, in a statement released by Ryanair, had said he was excited to return to the airline.
Malaysia Airlines could not be reached immediately for comment.
State investment fund Khazanah took private the carrier at the end of 2014 in the aftermath of the twin aviation disasters the same year.
Mr Bellew has brought a low-priced focus on costs to Malaysia Airlines, while at the same time strengthening its full-service proposition in a ferociously competitive market. He also improved in-flight meals and the company's online sales system by shifting to Amadeus, a merchandising platform. Bellew revealed little about his working relations with Khazanah but said he and his family were enjoying life in Malaysia.
"We are pleased to welcome Peter back to Ryanair". Ryanair could be facing big compension bills after a problem in the scheduling of pilots led it to cancel those flights.
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