Probably the wrong place for this question, but it would seem to me a lot would depend on whether you have checked luggage or not. If you have checked in online already then with Ryanair the online check-in process - on the very first screen - requires French citizens to provide a passport number or Carte d'Identité number and an valid expiry date. They'll be expecting to see that at baggage check in. If no checked luggage, then you may get away with flashing the expired ID card at the boarding gate, but you need to live with the consequences if they say its not acceptable - probably no travel at all. Incidentally ID cards initially issued in 2004 and 2005 are actually valid for 15 years, not 10 years, contrary to popular belief, and FR is aware of that. ID cards issued in 2003 or earlier are not now valid.

Ryanair accept driving licences but only for certain purely domestic flights.

Incidentally it is a requirement for all foreigners in Belgium to have valid ID on them at all times (unlike France). Belgians also need to have ID if more than 201 metres from their main residence. So you may also want to factor in what the police would say in the unlikely event that Ryanair contacted them.