Ryanair carried just over 4.1 million passengers in April, an increase of 19% on the same month last year.
Its latest passenger figures also showed a drop in its load factor - the percentage of seats filled - from 85% to 83%.
Chief executive Michael O'Leary blamed the fall in load factors mainly on the 21% growth in capacity, including the launch of three new European bases.
He said the doubling of UK passenger duties and higher airport charges at Stansted, as well as increased charges at Dublin Airport, had also affected the numbers and led to pressure on average fares.
Mr O'Leary said he expected this weakness to continue through the first half of the airline's financial year to the end of September.
