You can apply for a refund of the government taxes, but...All Ryanair fares are non-refundable - except in the case of a flight cancellation or a significant flight time change.
Can I apply for a refund for my unused flight?
All Ryanair fares are fully changeable flight/dates/times/routes and names (up to 4 hours prior to original flight departure) however fares, fees or charges are non-refundable.Government taxes may be refunded. Ryanair apply an administration charge per person for tax refund requests. If the refund amount due to the customer is less than the applicable refund administration charge then no refund will be made. Tax refund requests must be made within one month from the outward flight travel date.
I don't think you have to cancel it - you just don't check-in or turn up at the airport. Once the date has passed, if you qualify for a refund (and most don't), you can contact the airline by phone or email.
That's what I did a couple of years ago when a trip I'd booked fell through (football match cancelled, so I didn't travel). I didn't contact Ryanair in advance of the flight as I didn't see the point - and I didn't get any money back either!
you can contact the airline by phone or email.