What has this got to do with EASA?
The UK will brake with EASA, unless the negotiations end up with an agreement that includes UK membership in EASA. Currently there are several countries that are non-EU, but EASA members. Eg. Norway, Switzerland and Iceland.
EASA does not only certify new Airbus aircraft etc., but is the European authority for the air navigation system, air traffic management and all air and airport operations. There are lots of regulations and they also do inspections.
Compliance with EASA safety regulations is a must for operating in EU airspace.
For Ryanair and other airlines, it is not just UK domestic or flights with the EU that is at stake. Traffic management and navigation must be agreed upon, including for example how many flights EU airlines can fly into the UK, and who many flights UK airlines (and to which airports and slot time) can fly into the EU.
British Airways among other will also be affected. The UK and EU must agree on for example over fly rights and corridors. That is rights for aircraft from the UK (airline, aircraft models, slots) to fly into or over EU airspace, for example when flying to Asia, and of course the price for doing so (usually calculated based on MTOW).
For example Norwegian is not allowed to fly over Russia with their 787s. They take a 2 hour detour around in order to reach Bangkok. SAS on the other hand are allowed to fly over Russia with their A340, including flights to Japan and China.
BREXIT is way more complicated than many think, and Theresa May have signaled a hard exit from the EU.