Most airlines have no intention of paying EC261 and so they will make the process as difficult and cumbersome as possible. Many will just ignore you in the hope that you will simply go away. What you are "entitled" to is irrelevant and they will happily ignore this. The question is how to get some action going.
Enforcement agencies are here: https://ec.europa.eu/transport/sites/transport/files/2004_261_national_enforcement_bodies.pdf Technically you can complain there, but these tend to be large government agencies with spotty track records of actually doing something.
Your best shot is probably to keep writing to Ryanair with registered and legal sounding letters with due dates and the threat of interest, legal action and government escalation. These are not intended as a legal instrument (you would need a lawyer for this) but to convey the message that you are not going away peacefully. If you are lucky Ryan Air will eventually yield.
There are also third party companies that will fight the claim on your behalf for a sizable percentage of the compensation. They tend to have a good track record since they fight every claim to the end with full legal weaponry if required. The airlines know this and it's cheaper for the airlines just to pay.
EDIT Full test of the law is https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32004R0261
Concerning time it says in Article 8:
(a) - reimbursement within seven days, by the means provided for in Article 7(3), of the full cost of the ticket at the price at which it was bought, ...
That only covers the refund but not automatically the compensation and I don't think is specified anywhere.
In article 7(3) it only says
The compensation referred to in paragraph 1 shall be paid in cash, by electronic bank transfer, bank orders or bank cheques or, with the signed agreement of the passenger, in travel vouchers and/or other services.
So it specifies the means but not the timing.