text: In fact, you're confusing two things: administration (alternative and de facto by the majors of Irun and Hendaye) and the ownership (which is French AND Spanish all the year around). See https://en.wikipedia.org/wiki/Condominium_(international_law), A condominium [...] in international law is a territory [...] in or over which multiple sovereign powers formally agree to share equal dominium (in the sense of sovereignty) and exercise their rights jointly, without dividing it into "national" zones.
  In https://en.wikipedia.org/wiki/Pheasant_Island#Access, you see that naval forces are monitoring the area together.