The Irish airline pointed to a 6% fall in fares and increased fuel costs, which rose by almost 25%, as well as the impact of new pay deals with pilot and cabin crew unions, which saw staff costs soar by 28%.
But this guidance was "heavily dependent", O'Leary said, on peak summer fares, prices in the second half of the year, the absence of security-related events, and "no negative Brexit developments".
Profits for the entire group, which includes Laudamotion, would be flat - coming in the €750m to €950m range - mainly because the airline expects fares to stay low.
Pre-tax profit at the low-priced carrier fell nearly 30 per cent to €948m (£830m) in the year to March 2019, with CEO Michael O'Leary warning that profits for the coming year would be "broadly flat".
Russ Mould, investment director at AJ Bell said rising costs in the wider airline sector were impacting the company.
The group said it has delayed the delivery of its first five Boeing 737-MAX aircraft to winter 2019.
"We continue to have utmost confidence in these aircraft which have 4% more seats, are 16% more fuel efficient and generate 40% lower noise emissions", Ryanair said.
All B737 Max aircraft have been grounded worldwide since March this year, following two fatal crashes involving Lion Air and Ethiopian Airlines.
The fall in profits came even as revenue jumped 6% to €7.6bn and traffic grew by 7%.
"Short-haul capacity growth and the absence of Easter in Q4 led to a 6% fare decline, which stimulated 7% traffic growth to over 139 million (142 million guests including Lauda)", he said.
"Ancillary sales performed strongly up 19% to 2.4 billion euros (£2.1 billion), which drove total revenue growth of 6% to 7.6 billion euros (£6.6bn)".
Ryanair's board has also approved a €700m share buyback which will commence later this week and run over the next year.