Thirty-three Ryanair passengers were treated in hospital, some bleeding from their ears, after their Croatia-bound plane lost cabin pressure and made an emergency landing in Frankfurt, German police have said.

Oxygen masks were released on the Ryanair flight FR7312 from Dublin to Zadar in Croatia when it lost cabin pressure and diverted to Frankfurt Hahn airport, Ryanair said in a statement.

"In line with standard procedure, the crew deployed oxygen masks and initiated a controlled descent," the airline statement said.

A log on flightradar24.com showed the flight descending from 37,000 to 10,000 feet over a seven-minute period 80 minutes into the flight.

Ryanair said the plane "landed normally and customers disembarked, where a small number received medical attention as a precaution."

A spokesman for German police said 33 of 189 passengers were hospitalised, some bleeding from their ears.

The spokesman said some were still receiving treatment today.

German air accident investigator BFU, responsible for investigating the incident, said its team was heading to Hahn airport to secure the cockpit voice recorder and flight data recorder and to interview crew and passengers.

The BFU spokesman declined to comment on the possible cause of the incident.

Ryanair said a flight had left Frankfurt to Zadar on Saturday morning. Police said some passengers had decided not to continue with their journey.

Ryanair said it had agreed to pay for hotels for the affected passengers but said there was a "shortage of available accommodation."

Minerva Galvan, a Spanish citizen who was on board the flight described the moment it happened, saying they heard a crack and suddenly there was no air in the cabin.

Speaking to RTÉ News she described how around 90 minutes into the flight from Dublin the oxgyen masks came down and it was straight into an emergency landing.

She said the descent happened quickly. "Some people were bleeding from the ears and nose. Others were nervous but no one panicked. Not even the babies cried," she said.

Ms Galvan commended the cabin crew for their professionalism during the emergency landing but said on arrival at the airport "they were forgotten about" by the airline.

She said many passengers had to sleep on the floor while some children were given a room with some beds.

She added around 30 people who suffered nose and ear bleeds and headaches, including her Croatian boyfriend, were taken to hospital. She was not allowed to travel with him to the hospital.

He has since been released from hospital but is not allowed to fly today due to the injuries he sustained during the loss of air pressure, so they are searching for an alternative means of transport.

The pair were travelling to Croatia to visit her boyfriends' family.

She said some of the other passengers were travelling home ahead of Croatia's World Cup final tomorrow and many others were holidaymakers, including families.

Some of the affected passengers were due to take an alternative Ryanair flight from Frankfurt to Zadar at 9:30am this morning.

Ms Galvan said around 18 passengers are continuing their journey by bus.

The bus transfer was arranged by Ryanair to facilitate customers who could not take the rescheduled flight to Zadar this morning or who did not want to. 

She said two families are among the passengers taking the bus journey, which they have been told will take approximately 18 hours.