Ryanair said fares slumped by 17% as competition among European rivals remained strong, adding that the Brexit vote also helped eat into profit.
Europe's largest low-cost airline budget airline said third-quarter profit slipped 8% to €95m (£82m, $102m) in the third quarter of its year as average fares tumbled to €33 a passenger.
But it stood by its previous guidance that annual post-tax profit would be in a range from €1.3bn to €1.35bn, although it said that guidance depended heavily on the absence of "security events" affecting bookings in its final two months of the year. It described its outlook for the rest of the year as "cautious".
The business said traffic jumped by 16% to 29 million passengers in the period as it "stimulated demand through lower fares" at a time when many European carriers also added routes in a bid to grow market share.
Ryanair chief executive Michael O'Leary said: "We continue to grow capacity, new routes and bases, at a time when other EU airlines are also adding capacity, and accordingly the price environment remains weak."
The business added it had seen a fall in profit due to "a sharp decline" in sterling, which has fallen some 16% against the dollar since the June vote to leave the European Union.
The airline launched 95 routes and five bases during the period in Bucharest, Hamburg, Nuremberg, Prague and Vilnius. Next month it is set to open two further bases in Frankfurt and Naples.
The carrier competes against such rivals as Luton-based easyJet and Germany's Air Berlin.