Ryanair CEO Michael O'Leary is a fan of the Boeing aircraft company. In fact the Irish airline magnet just purchased 175 of Boeing's next generation 737 jets for $15.6 billion.

'Look at the economics of the 737, the Boeing has 189 seats. The European Airbus A320 has 180 seats. Those nine extra seats when you're flying them eight times a day, 365 days a year are a compelling competitive advantage for Boeing,' O'Leary told CNBC.

'It's why Ryanair has a lower competitive cost of any other airline in Europe, most of whom fly Airbus.' The average flight time on Ryanair’s European routes is about 90 minutes to two hours at the cost about $40 a ticket — half the average cost of one on Southwest Airlines.

O'Leary often has seating room on his mind, once saying he wanted to take the back bathrooms out of his planes. But he told CNBC that regulators won't let him, although if he could achieve the six extra seats he wanted, ticket costs for all his passengers would be 'lower by another 5 percent.'

O'Leary has looked into creating a 'standing-room only' section on his planes like on trains and buses, because one day there'd be so much room on board that 'passengers could fly for free with the airline making money selling snacks and the other stuff.'

O'Leary's order, signed in New York, will allow Ryanair, Europe’s leading low-cost carrier, to increase its fleet to 400 planes. Ryanair’s capacity will reportedly grow by 25 percent to 100 million passengers a year by 2018.

'This is a huge deal for the Boeing Company,' Ray Conner, the president and chief executive of Boeing’s commercial airplane division, said in a news conference in New York. 'We’ve been talking about this for quite some time.'

Ryanair is one of the few airlines with an all-Boeing fleet. Meanwhile O’Leary said the airline saw significant growth opportunities in Europe as other carriers cut back service.

Ryanair’s last big order with Boeing was in 2005, and its last plane was delivered at the end of 2012. The new planes are scheduled to be delivered between the end of 2014 and 2018.

'There is a pressing need for us,' O’Leary said at the news conference. 'This makes it easier for Boeing to negotiate with us and to squeeze us for slightly higher prices.'

O'Leary joked at the signature ceremony about the price he had paid. 'Fifteen billion dollars, and all I get is a pen,' he said.