Holidaymakers returning from Budapest on Saturday night started falling ill and fainting in their seats.
Garage manager Shaun Pinkerton, 27, told how he was one of four passengers to collapse.
"Everyone was panicking and we still don't really know what happened,” he told a national newspaper. "It makes you not want to fly again. It was the flight from hell.”
He said two other passengers - one as young as 17 - needed CPR after collapsing.
"I was knocked out and needed an oxygen mask. There was something seriously wrong with that flight.”
Cabin crew had to wheel out oxygen tanks to help the stricken passengers - scattered throughout the cabin.
Shaun, who had just enjoyed a four-day break with pals, said: "The fire alarm was going off while we were boarding but the flight took off as planned.
"We got up in the sky and about an hour in the first person collapsed. She was only 17 and needed CPR.
"Then the second person went down, then me, and then a fourth.
"There were people standing in aisles giving CPR and they had huge oxygen tanks out.
"I was knocked out and needed an oxygen mask. Then I went into shock and was total shaking."
Shaun, who has placed a formal complaint with the budget airline, said the ordeal has left him terrified to fly again.
He added: "I'm a young guy, I fly all the time and I'm always fine. There was something seriously wrong with that flight, folk were worrying there wasn't enough oxygen.
"I'm due to go to Poland in three weeks, and Madrid three weeks after that but I don't want to step foot on a plane ever again. It was like something out of a movie."
Ryanair has been approached for comment