After Brexit, UK nationals are subject to the same Schengen entry conditions as e.g. US, Canadian or Australian nationals (to name just a limited number of comparable examples). Without a visa, these passport holders are only allowed a total of 90 days in the Schengen zone within the last 180 days. I don't know how strictly they look for this - it can be difficult, if not impossible, to determine from the stamps in the passport in some cases - but this should become easier when the EU Entry/Exit scheme finally comes into force [currently planned for October 2025 but it has been delayed multiple times already].

UK passport holders are perhaps more likely to have exceeded this 90/180 day rule than other passport holders - due to proximity if nothing else.

All airlines - including Ryanair - will therefore want to see a UK national's passport - whether or not they have or require a visa - before accepting them for a flight to any Schengen state.

And soon, all third party nationals - including UK passport holders - will have to hold an ETIAS before travelling to a Schengen member state (just as happened earlier this year for Schengen state nationals travelling to the UK; they are now required to obtain an ETA before travel - Electronic Travel Authorisation (ETA) factsheet – 9 April 2025 – Home Office in the media)