Reading the text in English of regulation 261/2004, paragraph 12 states:
The trouble and inconvenience to passengers caused by cancellation of flights should also be reduced. This should be achieved by inducing carrier to inform passengers of cancellations before the scheduled time of departure and in addition to offer them reasonable re-routing, so that the passengers can make other arrangements. Air carriers should compensate passengers if they fail to do this, except when the cancellation occurs in extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken..
Article 5, paragraph 3 on the subject of cancellation states
An operating air carrier shall not be obliged to pay compensation in accordance with Article 7, if it can probve that the cancellation is caused by extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken.
Getting Ryanair to pay compensation beyond the cost of the ticket is likely to involve a court - perhaps small claims or magistrate. In a court, I suspect that an airline would claim the eruption of a volcano for the first time in 200 years causing the closure of much of European airspace to be an extraordinary circumstance. It would be difficult to show to a court that this is not an extraordinary circumstance.