Ryanair will have no effect on Jet2, this has been proven already with Ryanair's LBA-GRO/ALC routes. Jet2's LBA-BCN/ALC has seen no dip in passenger figures average load factors averaging 90%, often Jet2 carrying more pax on a 733 than Ryanair taking on a 738.
I think it is more likely that MAN will suffer due to Ryanair increasing the amount of options available at LBA. Interesting to see that Ryanair currently have no services on sale from either Doncaster or Teeside after October 2009. I fear these two may be the big loosers in the Ryanair base at Leeds.