Ibiza is a town in the Balearic islands, which maybe part of Spain, but are not in the Spanish speaking part of Spain.
The islands are off the coast of the northeastern corner of Spain and like that corner of mainland Spain the native language is not Spanish, but Catalan ( a Romance language like Spanish, but more akin to French than to Spanish).
The main city of the Catalan region is Barcelona, which the locals pronounce as "Barth-a-lona".
So the question is not "how do you say it in Spanish?", but "how do you pronounce it in Catalan?".
The answer?
Beats the heck outa me.