The Aviation Authority has ordered Ryanair to check oxygen masks on 21 aircraft following a complaint that some masks were old and not functioning properly.
Ryanair must report back to the Aviation Authority when the checks are carried out on their fleet of Boeing 737 200s.
The 15 other aircraft in Ryanair's fleet were new Boeing 737 800 planes and did not need to be checked, according to Ms Lilian Cassin spokeswoman with the Aviation Authority.
Staff at the Aviation Authority investigated the complaint and found the head cord on masks above three seats on a Ryanair aircraft were "slightly old or frayed", Ms Cassin said.
A spokeswoman for Ryanair said the airline had been aware of the problem which came to light on a flight from London to Glasgow on March 2nd. The checks were being made when the authority ordered them to be carried out, she said.
Ms Cassin said the checks were ordered as a safety precaution but it was not necessary for the head cord to fit the masks on to the face.
"It is not guaranteed a perfect seal. In some instances you may have to hold them up and that's acceptable," Ms Cassin added.
The Aviation Authority received a call from a member of the BBC 2 Working Lunch TV programme whose colleague had been on a Ryanair aircraft when the masks had dropped down and had noticed problems with them.
The masks dropped down as a result of a technical problem, the Ryanair spokeswoman said.
Oxygen masks are checked about every nine months and detailed examinations are done every three years. A Ryanair maintenance company carries out the checks which are approved by the authority.