Well done for landing the interview.
Ryanair are known for their no-nonsense approach to travel, and are very friendly in terms of how they operate. However, do not take this attitude as an excuse to be less formal at your interview. Dress smartly unless instructed otherwise, be polite, and answer questions as thoroughly as possible.
In a group interview, it is important you stand out, not as the cocky arrogant one who stepped on others to win, but the one who worked well as a team, listened to others, and helped others to get the best result they could. Group interviews can include designing a presentation, building something as a group, or solving problems in a limited time.
Good luck!