Most of Ryanairs international flights depart from Sat 3 @ STN (gates 40-59) which means a 10 min walk from check-in.
Gates 40-50 are in Sat 3 itself whilst 51-59 are in the lower level (FCF) at the very end of Sat 3.
The other international flights (FR) depart from Sat 2 which is the 2nd stop on the train. This takes you to the upper level of Sat 2.... the lower level being domestic.