Budget airline Ryanair carried 3.26 million passengers in August, up 27% on the same month last year.
But the load factor - which measures the amount of seats filled - fell back slightly from 92% to 91%.
Ryanair said the increase in passenger numbers meant this was the first month it had carried more people than British Airways. Earlier, BA reported that it carried 3.1 million passengers in August, down nearly 4% on the same month last year as its operations at Heathrow were paralysed by wildcat strikes sparked by job cuts at caterer Gate Gourmet.
Shares in Ryanair closed up 14 cent at €6.74 in Dublin.
