You can add a checked-in bag by modifying your booking online up to 2 hours before check-in, or at the baggage drop counter at the airport. The cost is 25 GBP/EUR if done upon booking the flight and 40 GBP/EUR if done after booking the flight. In general, it is cheaper to do this online and not at the baggage drop counter. Quoting from the linked website:
Steps to add
Bags can be purchased either when making the initial booking or can be added to a reservation via the My Booking facility up to 2 hours before the scheduled flight departure time.
The 20kg check-in bag costs €/£25 if purchased while booking the flight, and costs €/£40 after the booking is made.
Checked bags are non-refundable, are charged per person, per one way flight, and cannot be swapped between passengers or changed to other bag options. *Higher bag fees apply when checked bags are purchased via a Ryanair call centre, at airport ticket desks, during peak periods and on selected routes.