Europe's largest budget carrier previously owned a 75 percent stake in Laudamotion. Former Formula One racing champion Niki Lauda, who last year bought back and re-branded the airline he founded, gave Ryanair the option to buy the whole carrier.

"Laudamotion is now a 100 percent-owned subsidiary of Ryanair Holdings plc," Laudamotion said in a statement. It detailed plans to grow rapidly in the coming years, to 7.5 million passengers and 30 aircraft in 2021 from 4 million passengers and 19 aircraft this year.

At a news conference at Vienna's main airport, Ryanair Chief Executive Michael O'Leary declined to disclose the price his company paid for Laudamotion.

The deal for the last stake was completed on Dec. 31 but had nothing to do with Lauda's brief return to hospital shortly afterwards, O'Leary said. Lauda will stay on as chairman of Laudamotion's board.

"Niki has great experience in the airline industry, particularly in the airline industry in Germany and in Austria," O'Leary said when asked about Lauda's role. "He knows all the players. When we were buying it (Laudamotion) he had access to the various ministers in Austria, which we didn't have."

(Reporting by Francois Murphy; editing by Jason Neely and Emelia Sithole-Matarise)