Some passengers on the Dublin to Croatia flight received medical attention when it landed in Frankfurt on Friday, with reports of a number of people taken to hospital.
The airline said there was a shortage of accommodation for passengers who had to wait until Saturday morning for another flight.
A passenger reported on Facebook that the air went cold and the oxygen masks dropped, adding that there was a “dive of 3000 metres in less than one minute”.
He said a lot of people complained about sore ears, but praised the pilots for doing an “excellent job”.
The Tagesspiegel newspaper reported that 33 passengers were taken to hospital, adding that Federal Police said passengers were complaining of “headache and earache and suffered from nausea”.
A Ryanair spokesman said: “This flight from Dublin to Zadar [July 13] diverted to Frankfurt Hahn due to an inflight depressurisation.
“In line with standard procedure, the crew deployed oxygen masks and initiated a controlled descent.
“The aircraft landed normally and customers disembarked, where a small number received medical attention as a precaution.
“Customers were provided with refreshment vouchers and hotel accommodation was authorised, however there was a shortage of available accommodation.
“Customers will board a replacement aircraft which will depart to Zadar this morning and Ryanair sincerely apologised for any inconvenience.”