Ryanair is an all-Boeing operator and launched the higher capacity 737 MAX 8 in late 2014 with an order for 100 airplanes.
Leahy, speaking at his last big air show before retiring later this year, said he had expected the new Boeing plane to make more of a splash.
Boeing has landed a string of new plane deals while Airbus has won lucrative promises at the Paris Air Show, as both plane makers stressed the growing importance of digital technology in aviation.
"We have been impressed by the remarkable development of Ruili Airlines over the past three years", said Ihssane Mounir, senior vice president, Global Sales and Marketing, Boeing Commercial Airplanes.
Airbus immediately responded Monday with an order for 100 of its popular A320neo planes from leasing firm GECAS.More news: Portugal, Mexico, Russia tussle for semi-final berths
Boeing says it is increasing production of its 737 range from 42 to 47 planes a month by the end of 2017 and will reach 57 a month by 2020 to meet demand. Aercap now has 100 737 MAX airplanes on order. With 230 seats, the 737 MAX 10 is Boeing's answer to the Airbus A321neo, which had been outselling the smaller MAX 9 at a rate of five-to-one.
"We are very excited to announce our conversion of some of our 737 MAX 8s to 737 MAX 10s", said Peter Bellew, Managing Director and Chief Executive Officer, Malaysia Airlines.
"By placing a firm order with Boeing as well as contracting additional aircraft from our strategic partner Air Lease Corporation, Blue Air will incorporate 20 ultra-modern aircraft into its fleet", Gheorghe Racaru, CEO of Blue Air, said.
Once again, the bulk of Wednesday's contracts involved single-aisle airplanes, and again Boeing's 737 Max 10 drew headlines, leading with a 10-aircraft MOU with Xiamen Airlines worth $1.2 billion.
"Many airports are running out of capacity and for those airports this is a flawless aircraft", said Ajay Singh, chairman of low-priced Indian airline SpiceJet, which signed a provisional deal to buy 40 MAX 10s.
But analysts expect demand at the June 19-25 event to fall short of recent years, and some aviation companies themselves have cut back staff and hospitality at the show. Established carriers are also swapping out older, less efficient jets for new versions and low-priced airlines are rapidly expanding their fleets.