If you paid over £100 with a credit card, use a legally binding Section 75 claim via your credit card issuer. If you paid with a debit card, talk to your bank about chargeback. Banks might try to get rid of you by saying that a voucher is sufficient because the bank can't be bothered with the hassle, but it's worth persisting
https://www.moneysavingexpert.com/sh...our-purchases/
https://www.moneysavingexpert.com/sh...rd-chargeback/
Ryanair used to keep all their assets (apart from aircraft) solely in Ireland so difficult to enforce any order by a court in the UK. While the UK was a member of the EU, it was possible to use the pan-EU European Small Claims procedure in a court in the UK which was enforceable in Ireland. I believe this is still valid in the Brexit transition period - Google is your friend on this
https://www.gov.uk/recover-debt-from-eu
Alternately, go for the standard small claims court in the UK (simple procedure in the sherriff court in Scotland) and, if you have enough patience, try petitioning the High Court for the right to arrest one of Ryanair's aircraft while it's in the UK. Expensive route to take, but guaranteed to get the result you want very very quickly - along with some newspaper headlines
Final option is to get small claims court to confirm your opinion, and then have a moan to the CAA and see what happens. Ryanair are not a member of an Alternative DIspute Resolution scheme. The CAA will expect you to have correspondence proving you tried to resolve the matter directly. The UK CAA will get involed only in cases where a flight departed the UK - if the flight was to depart (for example) Madrid, they will direct you to their Spanish counterparts
I imagine Ryanair are betting that unless the amount is large, most people will decide to just accept a voucher, and then forget to use it within 12 months, by which time the voucher expires - so free revenue for Ryanair without even having to fly you anywhere !