To add to Dereko1969's advice - Munich airport to Munich Ostbahnhof is about 30 minutes by train with trains leaving every 15 minutes .. when you get the train at Munich airport be careful which one you get there's other trains (I can't remember if it's line S1 or S8 you should get) that also go to Ostbahnhof but via a much longer route .. you want the ones that go to Ostbahnhof directly , and not via central Munich first.
Of course if it's Ryanair's definition of Munich (Memmingen Airport) this information isn't relevant!