The airline said it would examine the ruling before deciding on an appeal.
The tribunal ruled that Vanessa Redmond was unfairly dismissed.
However, it described her breaches of procedures as serious matters warranting disciplinary sanction.
Ms Redmond had been dismissed following an allegation she blocked off seats on a plane flying from Dublin to Durham in May 2005 and fell asleep while reading a book.
The EAT has ordered Ryanair to re-hire Ms Redmond within the next five weeks.
She had admitted blocking off the seats, but other Ryanair staff gave evidence that this was the norm.
The tribunal said that evidence that Ms Redmond was sleeping may at a stretch fall short of proving conclusively that she was asleep.
However, the EAT said that it remained uncontroverted by evidence that she had failed to carry out security duties. It called these serious matters warranting disciplinary sanction.
But the EAT found that Ryanair's disciplinary process had shortcomings and that she had been denied fair procedure and natural and constitutional justice.
This rendered her dismissal unfair.
Both sides had argued that financial compensation would be the appropriate remedy.
