I'm not sure if this is common knowledge, but isle and island are not related, despite the similarities in phonology and semantics. Isle is the direct descendant in French of the Latin insula. In typical French fashion, most of the sounds disappeared, though one letter stuck around as a silent letter in English (in modern French, it's simply île).
The Old English form of island was igland (the g was pronounced as a y after an i). It would've sounded something like ee-land before the Great Vowel Shift. The ig bit actually descends from the Proto-Indo-European *akwa (like the Latin aqua), meaning "water." In Proto-Germanic, it became *ahwa. A couple vowel changes and spelling changes later, we end up at Old English. Fast forward several hundred years more, and someone decides that iland (as it was spelled in Early Modern English) must've come from the French isle + land, so the assumed silent s was stuck in. And we've been stuck with it ever since.