The landmark figure was achieved on the back of a bumper December, which saw a 7.5 million customers use the carrier – a rise of 25%. The load factor rose by 3 percentage points to 91%.
Ryanair said the rise came on the back of lower fares following the terrorist events in Paris and Brussels.
Kenny Jacobs, chief marketing officer, said: "We are pleased that over 101m international customers chose to fly with Ryanair in 2015, as we became the first airline to reach this international traffic landmark.
"This is thanks to our load factor active/yield passive pricing, our expanding route network and the continuing success of our "Always Getting Better" customer experience programme, which continues to deliver stronger than expected traffic and load factors, albeit at lower fares."
