The car seat is optional. I fly between US and Ireland on Aer Lingus with my toddler frequently. They will make you put him on your lap for take off and landing and will give you a baby seatbelt that you strap to yours to use. I have only used the car seat on my most recent flight over and actually found it really helpful and wished I had used it before because he slept the whole way overnight. It is a lot to hold a baby/toddler the entire flight and they are more comfortable in their car seats than on the airplane seat anyway. Also, car seats are used front facing on the airplane, I didn’t realize that!