A Schengen visa extension is possible in cases where
... a visa holder has provided proof of force majeure
or humanitarian reasons preventing him from leaving the territory
of the Member States before the expiry of the period of validity of
or the duration of stay authorised by the visa...
See here for more:
Schengen visa extensionHeart problems might qualify in this category, however, I see several problems in your case. A) Your French visa is already expired; an extension needs to be applied for while the visa is still valid. B) The fact that you can fly to Ireland to see your cardiologist suggests that your health problems are not serious enough to prevent you from leaving the territory.
It appears to me that chances of your visa extension application being denied are considerable.