Extra Six Nations Paris flights from ?199 return
Why on earth would you expect that to be 19.99 return? It is a specific event creating a huge spike in demand based around being one of many thousands of people who will only accept being in one place at a particular time. Do you expect East Coast trains to offer its cheapest fares every time England play Scotland at Murrayfield? If that game was moved to another time, would they lay on extra trains for it? No - because they can't.
Be glad that Ryanair can put on extra flights for this game, appreciate that it is a market led situation, and you can't blame the devil from Mullingar for the original game being cancelled in the first place.
The only scenario when people should whinge is if the flight got delayed and passengers missed the match. There will be no extra compensation for that. But that's a risk you take - be glad you are in a financial position to go to a game everyone else watches on TV. And if Ireland lose and your gambling stake goes down the toilet, you can't blame Ryanair for that either.