The low-cost airline posted profits after tax of €243 million (£219m) in the three months to June, down from €309m one year earlier.
Ryanair said that costs rose by 19 per cent as its fuel bill soared 24 per cent higher and staff costs increased by 21 per cent.
Passenger numbers rose 11 per cent to 42 million, however the carrier's average fare price dropped 6 per cent to €36.
The group said its two weakest markets were Germany, where it was affected by Lufthansa's acquisition of Air Berlin, and the UK, where Brexit concerns weighed "negatively on consumer confidence and spending".
Chief executive Michael O'Leary said profits after tax for the year remains unchanged at between €750m to €950m. He added: "The current weak fare environment has continued into Q2 and we expect first-half fares to be down approximately 6 per cent."