Does Ryanair lease any aircraft? Does Ryanair lease any of its aircraft?

And if they do, could the end of the lease period coincide with the start of the low passenger periods? E.g. during the autumn to spring months when holiday traffic is slow, there may be a need to reduce the fleet size.
Ryanair mainly buys their aircraft. Part of their ability to offer cheap flights comes from buying aircraft in bulk and generally only operating a single type of airframe. 

They generally buy new Boeing aircraft operating primaraly the 737 series.  


  ...had agreed to buy five more Boeing 737-800 aircraft, taking its order
  book to 180 new Boeing planes worth over $16 billion.


Planespotters.net has a nice list of the aircraft in Ryanair's fleet, with the exception of this -700 the rest of the aircraft only show registration to Ryanair as an owner under owner history.