I traveled home on Aer Lingus trans atlantic last year with our 4 month old. It was a rear facing carseat, we just turned it around. no problem. It's the type that clicks into a reciever in your car, but the aircraft seatbelt fits over it easily. This year we're travelling back with a larger forward facing car seat. It's FAA approved. I plan on slipping the aircraft belt through the back, as a car seatbelt would.
You may still be required to hold the infant on your lap for take-off and landing. Crew will provide a special belt for this.