Ryanair flight FR-449, a scheduled service between Liverpool and Dublin, was flying at a height of 18,000 feet when the flight crew initiated the emergency descent.
The flight was about 55 miles east of Dublin when the crew declared an emergency.
The crew carried out a "controlled descent" to 10,000ft, initially reporting problems with cabin pressure. The flight was then cleared to descend to 6,000ft, where the crew entered a holding pattern for about 20 minutes.
Other aircraft, also on approach to Dublin, were advised of the emergency, with some being placed in a queue to facilitate the troubled jet.
The Boeing 737-800 aircraft made a normal and safe landing about 30 minutes after the emergency was declared. Airport emergency fire and medical crews were on standby for the plane when it touched down.
A passenger told the Aviation Herald website the captain announced over the public address system: "Emergency descent. Emergency descent."
The flight crew also deployed the overhead oxygen masks in the cabin, while cabin crew members instructed passengers on how to use them.
Another passenger said: "The head steward was excellent — calm, firmly instructing other staff what to do. However, one of the other cabin crew totally lost the plot and was crying, shaking, whimpering. I appreciate she’s only human, but her behaviour really frightened the passengers and basically made us think: ‘Well, if the air hostess is this scared, things must be really bad’."
A Ryanair spokesman said: "Flight 449 experienced a depressurisation on approach to Dublin. In keeping with standard procedures, the crew triggered the oxygen masks and initiated a controlled decent to an unpressurised level. The aircraft landed normally and taxied to the terminal where all passengers disembarked safely."
It has also been reported that a number of passengers appeared to be shocked and were cared for by paramedics. However, the airline has not commented on this.
The AAIU has confirmed that: "an investigation is under way."
This appeared in the printed version of the Irish Examiner Tuesday, May 24, 2011