Category A: less than 169 km/h (91 kt) indicated airspeed (IAS) Category B: 169 km/h (91 kt) or more but less than 224 km/h (121 kt) IAS. Category C: 224 km/h (121 kt) or more but less than 261 km/h (141 kt) IAS. Category D: 261 km/h (141 kt) or more but less than 307 km/h (166 kt) IAS.
Configured in a single-class layout with 189 seats, Ryanair's Boeing 737-800s are the mainstay of the airline's fleet. Because of the low fares, I tend to think of Ryanair flights as like being on a bus in the air, as the seats do not recline and there is no in-flight entertainment.
So to your original question, a person who has a current type rated for required pilot flight crewmember duties on a Boeing 747 cannot perform the same duties on a Boeing 787, 737, etc. without being type rated on those aircraft as well.
Weight class is based are assigned by APO130 based on the TFMS observed aircraft codes. There are 6 categories -- (A) Heavy, (B) B757, (C) Large Jet, (D) Large Commuter, (E) Medium, (F) Small.