As Peter Rozovsky reported in the comments after the last post, "Island" and "isle" are etymologically unrelated. Crazy, right? Crazy, but true. "Isle" stems from the same source as the French île, as in Île de la Cité, the heart of Paris. It is connected to the Latin insula--you know, like in "insulate", or "peninsula". As the Online Etymology Dictionary has it, the "Ancients" guessed that it came from in salo--that which is in the sea.
"Island", though, has an Old English beginning. It went back through yland to the older iegland. "Land" is probably self-explanatory--ieg is more interesting. It's related to many water words, like "agua" and "aqua". Ieg shows up in British place names, where it means, "slightly raised dry ground offering settlement sites in areas surrounded by marsh or subject to flooding". It is a little bit of land upon the waters.
All right, all right, but where is that blasted "s" coming from? In either case? It seems to be that the s in island comes across from contamination from French, but how can that be when île didn't have an s in it either?
Students of French probably already have the answer. See that little cap over the i in île? That's what's known as a circumflex, mes amis. The circumflex represents a missing, silent letter--often an s. The French dropped a lot of 's's over time. What's interesting is that when the French Normans invaded England and came to stay for a couple few centuries, the English gamely added those words to their own language. Words like forest, hospital, feast. These words sounded like the Old French words. But in France itself, these words were undergoing a change in the vernacular. The circumflex was the monkly nod (monks being the ones writing stuff down at that time) to that vanished, silent s. Forest is forêt. Beast is bête. Paste is pâté. Here's the link that helped me understand all this.
And here's a discussion of more surprisingly unrelated words.
Like I always say--you never know where ignorance is going to take you...