Ryanair are sticklers for their rules and will charge if you step outside them. I read the below as if your checked baggage is a single item less than 15 kg and 81x119x119cm there are no extra charge other than that you have paid for one piece of checked baggage, but do not take my word for it.
"Checked Baggage is any item accepted for carriage in the aircraft hold, including bags, infant equipment (baby seats/booster seats/travel cot), musical instruments and sports, equipment etc. (mobility equipment and children's' pushchairs are carried free of charge and do not need to be prebooked). For health and safety reasons Ryanair does not accept for carriage any individual item exceeding 32 kilos or with combined dimensions of more than 81cms (height), 119cms (width) and 119cms (depth). This weight limit does not apply to mobility equipment.
Each passenger is permitted to check- in up to a maximum of 2 bags with a maximum weight of 15 kilos per bag (subject to the payment of the applicable checked baggage fees). Checked baggage can be purchased online at the time of booking or via the 'Manage My Booking' section of our website. Checked bags can also be purchased via a call centre up to 4 hours prior to the scheduled flight departure (subject to call centre opening hours) or up to 40 minutes prior to the scheduled flight departure time at the airport.
Checked baggage fees are charged per person/per one way flight. Discounted fees are available online.
No pooling or sharing of baggage allowances is permitted, even within a party travelling on the same reservation. Each checked bag is permitted a maximum 15kgs allowance. In the event that you are travelling with 2 checked bags please be aware that no pooling of individual checked bag allowance is permitted (each bag is permitted a maximum weight of 15 kilos). Any passenger exceeding their 15kg personal checked baggage allowance (per checked bag purchased) will be charged for the excess weight at the applicable rate per kilo.
At airports with self-service kiosks, you must have paid any checked baggage fees and/or excess baggage charges and deposited your checked baggage at the bag drop desk no later than 40 minutes before scheduled departure time.
CABIN BAGGAGE ALLOWANCE
Each passenger (excluding infants) is permitted to carry one piece of cabin baggage on board (free of charge). Strictly one item of cabin baggage is permitted per passenger (excluding infants). Your handbag, briefcase, laptops, shop purchases, camera etc must be carried within your permitted 1 piece of cabin baggage. It should weigh no more than 10kg and not exceed the maximum dimensions of 55cm x 40cm x 20cm. Due to security restrictions - certain items cannot be carried in cabin baggage - click here for details. For the safety and convenience of all passengers, cabin baggage must fit underneath the seat or in the overhead compartment. We reserve the right to cancel your reservation without refund and to deny you boarding if you arrive at the boarding gate with more than one item of cabin baggage or if that item exceeds the maximum dimensions.
EU/EEA cabin baggage security restrictions require that passenger cabin baggage containing any liquids, gels, pastes, lotions or liquid cosmetics MUST be presented at the airport passenger security point in a transparent re-sealable bag of no more than one litre capacity (e.g. 20cm x 20cm ziplock freezer bag). Only gels, pastes, lotions and liquid cosmetics in containers of 100mls or less will be permitted through the passenger security screening points at all EU/EEA Airports, please note there is a limit of one transparent resealable plastic bag per person."