What happens to Ryanair aircraft after they leave the airline?
Most of Ryanair's fleets is sold upon delivery and leased back from lessors. The lessors will then lease the aircraft out to another airline for as long as its safe for the aircraft to be flying.

An example for MSN 29921 would be:

EI-CSF    24. May 2000    Ryanair
C-FTCX    28. Mar 2008    CanJet
OK-TSJ    05. May 2015    Travel Service
C-FTCX    05. Nov 2015    CanJet
C-FTCX    04. Dec 2015    Air Transat
SP-ENO    30. May 2016    Enter Air


(planespotters.net)