ooohh, this another example of folk etymology, or perhaps wishful thinking. If Voltaire didn't make it up out of thin air (entirely possible) then he probably got it from the Spanish town, which is derived from the earlier, longer Arabic name Jazirat Shukr which means "Island of the Jucar (
River)". The Germanic element being referred to is OHG Ziari/Zeri, "splendid, beautiful", recorded only as the first element of a mere two names Zierhild and Zierolf; the possibly related Tir- (glory, honor) is hardly more common, although it is more widely recorded (
Saxon, English, Norse), but again, only as a first element, never a second.