I spent most of the night since the match working this out.
Cork - Nantes - Cork return charter is €28,300 on charter.com. That works out at €142 a skull if, say, 200 supporters went. The pitch is 16 miles from the airport, so a quick hop back to the airport after the match and there is time for a few pints at home. That price does not cover travel insurance, but my trip to Turners Cross today did not need to be insured, so why go to the extra cost. Travel to the pitch should be cheap enough using local taxi / bus / coach services.