Denied boarding by Ryan Air even though my passport is visa-free to the destination

Under regulation EC 261/2004, if you are denied boarding you are entitled to three things (see also Europa.eu):

• Compensation
• The right to chose between reimbursement, re-routing or rebooking
• Assistance

Assistance does cover accomodation but not PCR tests (it was written before the pandemic and doesn't cover visa fees either). The flight you accepted would seem to fall under the second bullet point but accepting that doesn't impact your right to compensation. That much is clear and relatively straightforward even if airlines are known to make difficulties to actually apply the rules.

The key question is whether EC 261 applies at all. It contains the following definition for “denied boarding”:

(j) "denied boarding" means a refusal to carry passengers on a flight, although they have presented themselves for boarding under the conditions laid down in Article 3(2), except where there are reasonable grounds to deny them boarding, such as reasons of health, safety or security, or inadequate travel documentation;

You did, in fact, have adequate travel documentation (and they accepted that fact) so I would conclude that you were denied boarding under that definition.

Concretely that means you are at least entitled to €400 (compensation for a flight over 1500 km).