Fiona Nangle (40), Latt Hills, Cavan, sued the airline over the accident when she fell on the vinyl floor surface in the forward cabin section shortly after take off of a Dublin-Warsaw bound flight on February 11, 2018.
As a result of the fall, the court heard, she suffered a spiral fracture to her right humerus, required surgery, was incapacitated for a number of months and needed assistance in daily tasks like looking after her children.
Ryanair denied her claims and argued she did not slip on de-icing fluid but tripped.
Mr Justice Alexander Owens rejected Ryanair's arguments.
He disagreed with its submission that she should have known, as cabin supervisor. about the hazards arising from de-icing fluid.
He also disagreed with the airline's suggestion that she was in some way the author of her own misfortune by failing to keep an eye out for the hazard.
Patricia Dillon SC, with Rory O'Reilly BL, instructed by Ronan Doherty solicitor, told the court Ms Nangle had been on the jump seat for take-off.
After the aircrew signalled to cabin crew they were free to leave their seats, while the plane was still climbing, Ms Nangle went to go to the galley to do paperwork.
Ms Nangle told the court there was a mat which passengers had walked over and as she stepped off the mat she slipped. She said she tried to grab the wall, but she fell in a heap on the floor and was in a lot of pain.
Mr Justice Owens said the plane, a Boeing 737, had undergone de-icing of its wings and tail on its return from an earlier Dublin-Birmingham trip and before its departure for Warsaw.
Evidence was given that the de-icing fluid was trekked into the aircraft prior to the first flight and again before the Warsaw trip, he said.
The judge estimated, assuming both flights that morning were full or nearly full, that some 450 passengers passed in the vicinity of where Ms Nangle fell that morning.
He said air carriers recognise the presence of de-icing fluid trekked in on footwear onto cabin floor surfaces is a recognised tripping hazard.
Ryanair itself had documented three reported instances of slips leading to injury of cabin crew in the two months before this accident, he said.
The day after Ms Nangle's accident, the airline issued an advisory email at the start of each shift that staff should remain vigilant at all times during times when de-icing occurs, he said.
If such a warning had been issued earlier, that would have been sufficient to discharge the duty to ensure, as far as reasonably practicable, her safety, the judge said.