The Friday night flight from Dublin to Zadar in Croatia was carrying 189 passengers when oxygen masks dropped down.
“The passengers complained of headaches and earaches and felt sick,” a police spokesman told the Germany press agency DPA.
In a statement quoted by DPA, the low-cost Irish carrier said the aircraft had suffered a sudden loss of cabin pressure, prompting the deployment of oxygen masks with the plane going into a controlled descent to Frankfurt-Hahn airport.
Some 30 people were taken to hospital and all were released during the morning after a check-up, airport officials said.
The passengers were due to continue their journey to Croatia aboard a second Ryanair plane, they said.