Passenger numbers at Aer Lingus Regional hit a record of just over 122,000 last month.
May was the 11th consecutive month of growth for the airline, which appears to have been completely unaffected by industrial action at Aer Lingus itself.
Aer Lingus Regional is operated under a licence by Stobart Air and serves a number of airports across Ireland, Britain and France.
The regional carrier flew almost 32,000 more passengers than in the same month last year.
The Heineken Cup final boosted traffic on its service to Cardiff during the month.
The opening of two Aer Lingus routes, to San Francisco and Toronto, has also seen Aer Lingus Regional double the number of passengers it is carrying from Britain into Ireland to connect on transatlantic flights.