Quoting from the 737 ( which Ryanair operate ) FCTM, page 2.5, 737-600 - 737-900, 'normal taxy speed is 20kts, adjusted for conditions. On long straight taxi routes, speeds up to 30 kts are acceptable, however at speeds greater than 20 kts use rudder pedal steering only. When approaching a turn, speed should be slowed to an appropriate speed for conditions. On a dry surface, use approximately 10 kts'.
This is straight from Boeing, 30kts on the emergency runway at LGW can feel quite slow, but on a narrower taxyway quite fast.
Reverse thrust, again I quote, 'During taxy.... the use of reverse thrust above reverse idle is not recommended due to the possibility of FOD.... Idle reverse thrust may be necessary on slippery surfaces to control speed while taxying'.