late 13c., ile, from Old French ile, earlier isle, from Latin insula "island," a word of uncertain origin.
Perhaps (as the Ancients guessed) from in salo "(that which is) in the (salty) sea," from ablative of salum "the open sea," related to sal "salt" (see salt (n.)). De Vaan finds this "theoretically possible as far as the phonetics go, but being 'in the sea' is not a very precise description of what an island is; furthermore, the Indo-Europeans seem to have indicated with 'island' mainly 'river islands.' ... Since no other etymology is obvious, it may well be a loanword from an unknown language." He proposes the same lost word as the source of Old Irish inis, Welsh ynys "island" and Greek nēsos "island." The -s- was restored first in French, then in English in the late 1500s.
"bright green precious stone," c. 1300, emeraude, from Old French esmeraude (12c.), from Medieval Latin esmaraldus, from Latin smaragdus, from Greek smaragdos "green gem" (emerald or malachite), from Semitic baraq "shine" (compare Hebrew bareqeth "emerald," Arabic barq "lightning").
Sanskrit maragata "emerald" is from the same source, as is Persian zumurrud, whence Turkish zümrüd, source of Russian izumrud "emerald." For the unetymological e-, see e-.
In early examples the word, like most other names of precious stones, is of vague meaning; the mediæval references to the stone are often based upon the descriptions given by classical writers of the smaragdus, the identity of which with our emerald is doubtful. [OED]
Emerald Isle for "Ireland" is from 1795.
1798, "of or belonging to the Isle of Man," between England and Ireland, earlier Manks (1620s), metathesized and contracted from Maniske (1570s) "of the Isle of Man," from Old Norse *manskr, from Man (from Old Irish Manu "Isle of Man") + suffix -iskr "ish." As a noun, 1680s as "native or inhabitant of Man," from 1670s in reference to the Celtic language spoken there (extinct since mid-20c). Manx cat, without a tail, is attested by 1843; the natural mutation arose among cats there and took root in a limited gene pool.