found this right on the ryan air website:
If you are travelling with an infant (aged 8 days up to and including 23 months), you can include them on your seat booking by making an infant reservation on our website or by calling your local reservations centre.
Newly born infants up to 7 days inclusive are not accepted on Ryanair flights.
For safety and insurance requirements, infants (aged 8 days up to and including 23 months) must be accompanied by a passenger aged 16 years or above and booked in the ratio of one infant per adult. Children aged between 8 days and less than 2 years as at date of travel cannot travel in their own seat and must sit on an adult's lap. (One infant per adult). N.B. Extra seats cannot be purchased for infants.
Click here for details of the charge applied for the carriage of infants.
If the infant reaches the age of 2 years prior to the return journey they must pay the applicable fare, taxes, fees and charges for that part of the journey. Proof of age may be required so please have the infant's valid photo-id available for inspection (or birth certificate if traveling on a route that does not require valid photo-id i.e. Ireland-UK Ireland, UK domestic or Italian domestic flights).
No baby/car seats are allowed in the cabin of the aircraft.
There is no checked baggage allowance for infants. However, one fully collapsible pushchair per infant may be carried free of charge. Additional infant equipment is charged at a fee per item. (Click here for details)
To ensure that all families travel at the best price and receive the best service we have introduced a number of services to make it easier for family travel:
Pushchairs can be used up to the entrance to the plane (provided that local conditions permit this). After landing your pushchair will be available at the aircraft steps.
Infant seatbelts are provided on all Ryanair aircraft and individual assistance can be provided by the cabin crew, if required to ensure that infant seat belt is used correctly.
Baby changing facilities are available on board.