The second syllable (also in Old Frisian alond, Middle Dutch eiland) was added later to distinguish it from homonyms, especially Old English ea "water" (see ea). As an adjective from 1620s.
Spelling modified 16c. by association with similar but unrelated isle. Similar formation in Old Frisian eiland, Middle Dutch eyland, German Eiland, Danish öland, etc. In place names, Old English ieg is often used of "slightly raised dry ground offering settlement sites in areas surrounded by marsh or subject to flooding" [Cambridge Dictionary of English Place-Names].
Island universe "solar system" (1846) translates German Weltinsel (von Humboldt, 1845). An Old English cognate was ealand "river-land, watered place, meadow by a river." Related: Islander.