insolation < Latin sol, solis m "sun"

insulation < Latin insula, -ae f "island" (apparently nobody knows where this one comes from)

isolation < French isolation < Italian isolare < isola < Vulgar Latin *isula < Latin insula, -ae f

Spanish aislamiento < aislar < isla < Vulgar Latin *isula < Latin insula, -ae f

Oh and the English island never had an s sound, but is spelled like that because of confusion with isle, which is an unrelated borrowing from Old French (île in modern French, with the diacritic signifying a lost s which was apparently already questionable at the time it was borrowed), ultimately also from Latin insula.

> Oh and the English island never had an s sound, but is spelled like that because of confusion with isle

So the German cognate (I assume) Eiland probably hasn't had one either. Makes sense, since the Nordic Eya / Øy / Ö never felt like they should, and they must be just northern variants that lack the -land (literally the same in English) suffix.