"I think as the U.K. largest airline, Ryanair has played a leading role in the campaign, and I think one of the things that U.K. citizens can understand that Europe has delivered for them has been the low-fare travel revolution," he said.
U.K. citizens are scheduled to vote Thursday on whether they want the country to leave the European Union.
O'Leary said a Brexit would be damaging to the European and U.K. economies for the next two-to-three years. "There'd be huge uncertainty while Britain tries to negotiate an exit out of the single market and tries to replace it with a whole series of trade deals, which won't get done."
Financial markets have been jittery ahead of Thursday's referendum, with U.S. stocks posting mild gains on Tuesday.