This is what the Ryanair website says:
https://www.ryanair.com/gb/en/#collapse-2
A Medical equipment baggage waiver letter is required to carry any medical equipment or supplies onboard a Ryanair flight.
Passengers' with pre-existing medical conditions who need to carry medical equipment/supplies in addition to their checked/hand baggage allowance should contact our
Special Assistance Line well in advance of travel.
On provision of a doctor's letter confirming their requirements, a medical equipment baggage waiver letter will be issued. Only items listed in the waiver letter will be accepted for carriage free of charge. Please ensure that these items are packed separately and available for inspection at the airport.
https://www.ryanair.com/gb/en/#collapse-14
No. Diabetic customers do not need medical clearance to travel provided their condition is stable.
If a customer is unable to administer their own medication then they should travel with an accompanying passenger who can help them.
You will also need to carry a doctor's letter confirming the requirement to carry syringes, needles and medication onboard the aircraft
Please note that syringes, needles and medication must not be disposed of onboard the aircraft.
I've only ever taken my stuff in my hand baggage and never bothered with a letter when flying Ryanair. It's never been a problem.