On Monday Europe's largest airline by passenger numbers reiterated a forecast made before the June 23 Brexit referendum that profit after tax would grow 13 percent in the year to end-March 2017 to between 1.38 billion euros ($1.51 billion) and 1.43 billion euros.
"I don't think any other airline in Europe will be delivering or forecasting that kind of profit growth, but all of the clouds on the horizon suggest there are significant risks to the downside in the second half of the year," O'Leary said.
Rival easyJet last week said it was unable to give an earnings forecast in the aftermath of Brexit, a deadly attack in Nice and an attempted coup in Turkey, while Germany's Lufthansa warned on profit.
O'Leary said in a statement Ryanair would offset the impact of a fall in the value of sterling since the Brexit vote by shifting some capacity away from Britain starting this winter.
"We will pivot our growth away from U.K. airports and focus more on growing at our EU airports over the next two years. This winter we will cut capacity and frequency on many London Stansted routes (although no routes will close) where we are already significantly ahead of our multiyear traffic growth targets," the investor update said.
He warned the company faced further disruption if Britain failed to secure access to Europe's Open Skies agreement, but that these risks would be manageable.
Ryanair flies 40 million of its 100 million-plus passengers a year to and from Britain and has its largest hub at London's Stansted Airport.