Yes, you can take a booster seat on a Ryanair flight, but there are specific rules regarding its use and transport. For children over the age of two, a backless booster seat cannot be used in the aircraft seat during takeoff, landing, or when the "fasten seatbelt" sign is on, as the aircraft's lap belt is designed to secure the passenger directly. However, you can bring a car seat or a "CARES" harness if it is FAA/EASA approved and fits within the 42.5 cm (16.7 inches) width of the plane's seat. If you are simply bringing the booster seat to use in a rental car at your destination, Ryanair allows you to carry two pieces of infant equipment per child (e.g., a stroller plus a booster seat) free of charge. These items are typically tagged at the check-in desk and stowed in the aircraft hold. If you try to bring a booster seat as a "carry-on" item into the cabin, it must fit within your purchased cabin bag allowance and be stowed in the overhead locker, as it cannot be placed under the seat in front of you.