Online Etymology Dictionary

irascibility (n.): 1701, from irascible + -ity.
irascible (adj.): late 14c., from Old French irascible (12c.) and directly from Late Latin irascibilis, from Latin irasci "be angry, be in a rage," from ira "anger" (see ire).
Irascible indicates quicker and more intense bursts of anger than irritable, and less powerful, lasting, or manifest bursts than passionate. [Century Dictionary]
irate (adj.): 1838, from Latin iratus "angry, enraged, violent, furious," past participle of irasci "grow angry," from ira "anger" (see ire).
ire (n.): c. 1300, from Old French ire "anger, wrath, violence" (11c.), from Latin ira "anger, wrath, rage, passion," from PIE root *eis- (1), forming various words denoting passion (source also of Greek hieros "filled with the divine, holy," oistros "gadfly," originally "thing causing madness;" Sanskrit esati "drives on," yasati "boils;" Avestan aesma "anger;" Lithuanian aistra "violent passion").

Old English irre in a similar sense is unrelated; it from an adjective irre "wandering, straying, angry," which is cognate with Old Saxon irri "angry," Old High German irri "wandering, deranged," also "angry;" Gothic airzeis "astray," and Latin errare "wander, go astray, angry" (see err (v.)).
ireful (adj.): c. 1300, from ire (n.) + -ful. Related: Irefully.
Ireland: 12c. in Anglo-Norman, a Germanic-Celtic hybrid, with land (n.) + Celtic Eriu (see Irish (n.)).
Irene: fem. proper name, from French Irène, from Latin Irene, from Greek Eirene, literally "peace, time of peace."
irenic (adj.): "promoting peace," 1854, from Greek eirenikos, from eirene "peace, time of peace." Earlier as irenical (1650s). Irenics is from 1834, originally a branch of theology.
irenology (n.): "study of peace," 1974, from Greek eirene "peace" + -ology. Related: Irenological.
Irgun: militant Zionist organization, 1946, from Modern Hebrew, literally "organization," in full Irgun Zvai Leumi "national military organization."
Irian: Indonesian name for New Guinea, said to mean literally "cloud-covered."
iridescence (n.): 1799, from iridescent + -ence. Related: Iridescency (1799).
iridescent (adj.): 1784, literally "rainbow-colored," coined from Latin iris (genitive iridis) "rainbow" (see iris). The verb iridesce (1868) is a back-formation. Related: Iridescently.
iridium (n.): silver-white metallic element, 1804, coined in Modern Latin by its discoverer, English chemist Smithson Tennant (1761-1815) from Greek iris (genitive iridos) "rainbow" (see iris) + chemical ending -ium. So called "from the striking variety of colours which it gives while dissolving in marine acid" [Tennant]
iris (n.): late 14c. as the name of a flowering plant (Iris germanica); early 15c. in reference to the eye membrane, from Latin iris (plural irides) "iris of the eye; iris plant; rainbow," from Greek iris (genitive iridos) "a rainbow;" also "iris plant" and "iris of the eye," a word of uncertain origin. Iris was the name of the minister and messenger of the Olympian gods (especially of Hera), visibly represented by the rainbow (which was regarded as the descent of a celestial messenger). From the oldest parts of the Iliad the word is used of both the messenger and the rainbow.

The eye region was so called (early 15c. in English) for being the part that gives color to the eye; the Greek word was used of any brightly colored circle, "as that round the eyes of a peacock's tail" [Liddell & Scott]. Another sense in Middle English was "prismatic rock crystal." Related: Iridian; iridine.
Irish (n.): c. 1200, "the Irish people," from Old English Iras "inhabitant of Ireland." This is from Old Norse irar, which comes ultimately from Old Irish Eriu (accusative Eirinn, Erinn) "Erin." The reconstructed ancestry of this derives it from Old Celtic *Iveriu (accusative *Iverionem, ablative *Iverione), perhaps (Watkins) from PIE *pi-wer- "fertile," literally "fat," from root *peie- "to be fat, swell" (see fat (adj.)).

From mid-15c. in reference to the Celtic language spoken in Ireland. Some Middle English forms of the word suggest influence of (or punning on) Old French irais, irois "wrathful, bad-tempered" (literally "ire-ous") and Irais "Irish."

Meaning "temper, passion" is 1834, American English (first attested in writings of Davy Crockett), from the legendary pugnacity of the Irish. Irish-American (n.) is from 1816 (as an adjective from 1820). Wild Irish (late 14c.) originally were those not under English rule; Black Irish in reference to those of Mediterranean appearance is from 1888.
Irish (adj.): c. 1200, Irisce, "of Irish nationality;" see Irish (n.). Irish stew is attested from 1814; Irish lace is from 1851; Irish coffee is from 1950. Meaning "Irish in nature or character," it is attested from 1580s, and until 19c. often meaning "contradictory." In later use often mocking or dismissive, such as Irish apricot "potato," Irish daisy "common dandelion."
Irishism (n.): 1734, from Irish (adj.) + -ism.
Irishman (n.): c. 1200, from Irish (adj.) + man (n.).
Irishry (n.): "people of Ireland, the Irish people conceived as a company or body," late 14c., from Irish + -ry.
Irishwoman (n.): c. 1200, from Irish (adj.) + woman (n.).
irk (v.): early 15c., irken, "to trouble (someone), disturb, hinder, annoy;" earlier "be lax, slow, or unwilling (in doing something); be displeased or discontented" (early 14c.); "be weary of, be disgusted with" (c. 1400); of uncertain origin.

Watkins suggests it is related to Old Norse yrkja "work." Middle High German erken "to disgust" also has been suggested. A Middle English adjective, irk, meaning "weary, tired, bored; distressed, troubled; troublesome, annoying," is attested from c. 1300 in Northern and Midlands writing; it is sometimes said to be from the verb, but it is older, and "Middle English Dictionary" says this is probably Celtic, and compares Old Irish arcoat "he injures," erchoat "harm, injury."
irksome (adj.): "bothersome, troublesome, annoying," early 15c., from irk + -some (1). Related: Irksomely; irksomeness.
Irma: fem. proper name; see Emma.
iron (n.): Middle English iron, iren, yron, from Old English iren, variant (with rhotacism of -s-) of isen, later form of isern, isærn "the metal iron; an iron weapon or instrument," from Proto-Germanic *isarnan (source also of Old Saxon isarn, Old Frisian isern, Old Norse isarn, Middle Dutch iser, Old High German isarn, German Eisen).

This probably is an early borrowing of Celtic *isarnon (compare Old Irish iarn, Welsh haiarn), which Watkins suggests is from PIE *is-(e)ro- "powerful, holy," from PIE *eis "strong" (source also of Sanskrit isirah "vigorous, strong," Greek ieros "strong"), on the notion of "holy metal" or "strong metal" (in contrast to softer bronze).

It was both an adjective and a noun in Old English, but in form it is an adjective. The alternative isen survived into early Middle English as izen. In southern England the Middle English word tended to be ire, yre, with loss of -n, perhaps regarded as an inflection; in the north and Scotland, however, the word tended to be contracted to irn, yrn, still detectable in dialect.
Right so as whil that Iren is hoot men sholden smyte. [Chaucer, c. 1386]
Chemical symbol Fe is from the Latin word for the metal, ferrum (see ferro-). Meaning "metal device used to press or smooth clothes" is from 1610s. Meaning "golf club with an iron head, 1842. To have (too) many irons in the fire "to be doing too much at once" is from 1540s. Iron lung "artificial respiration tank" is from 1932. The iron crown was that of the ancient kings of Lombardy, with a thin band of iron in the gold, said to have been forged from a nail of Christ's Cross. Iron horse "railroad locomotive" is from an 1839 poem. Iron maiden, instrument of torture, is from 1837 (probably translating German eiserne jungfrau). The unidentified French political prisoner known as the man in the iron mask died in the Bastille in 1703. In British history, Wellington was called the Iron Duke by 1832.
iron (v.): c. 1400, irenen, "to make of iron," from iron (n.). Meaning "shackle with irons" is from 1650s. Meaning "press clothes" (with a heated flat-iron) is recorded from 1670s. Related: Ironed; ironing.
Iron Age: 1590s, originally, as in Greek and Roman mythology, the last and worst age of the world; the archaeological sense of "period in which humans used iron tools and weapons" is from 1866 (earlier in this sense iron period, 1847).
Iron Cross: from German eiserne kreuz, instituted 1813 by Frederick Wilhelm III of Prussia, originally for distinguished military service in the wars against Napoleon.
iron curtain (n.): 1794, the name of a fire-protection device to be used in theaters, a literal iron curtain; see iron (n.) + curtain (n.).
The new and exquisitely beautiful theatre of Drury-lane has the peculiar contrivance of an iron-curtain to secure the audience from all danger, in case of fire on the stage. Miss Farren, in the occasional epilogue, delivered on opening this new theatre, pleasantly informs the spectators that, should flames burst out in the part appropriated to the representation, they may comfort themselves with thinking that nothing can be burnt but the scenery and the actors. ["The Monthly Review," June 1794]
From 1819 in the figurative sense "impenetrable barrier." In reference to the Soviet occupation of Eastern Europe, famously coined by Winston Churchill March 5, 1946, in speech at Westminster College, Fulton, Missouri, but it had been used earlier in this context (for example by U.S. bureaucrat Allen W. Dulles at a meeting of the Council on Foreign Relations, Dec. 3, 1945). The phrase had been used in the sense of "barrier at the edge of the sphere of influence of the Soviet Union" from 1920. During World War II, Goebbels used it in German (ein eiserner Vorhang) in the same sense. But its popular use in the U.S. dates from Churchill's speech.
iron-bound (adj.): late 14c., from iron (n.) + bound (adj.1). Figurative use from 1807.
iron-gray (adj.): Middle English, from Old English isengræg; see iron (n.) + gray (adj.). The color of freshly broken cast iron.
iron-on (adj.): 1959, from the verbal phrase, from iron (v.) + on (adv.).
ironclad (adj.): 1852 of knights, 1861, of warships, American English, from iron (n.) + clad. Figuratively, of contracts, etc., "very rigid or strict, allowing no evasion or escape," from 1884. As a noun meaning "iron-clad ship," it is attested from 1862.
ironic (adj.): 1620s, "pertaining to irony," from Late Latin ironicus, from Greek eironikos "dissembling, putting on a feigned ignorance," from eironeia (see irony). Related: Ironical (1570s); ironically.
ironing (n.): "act of pressing and smoothing clothes with a heated flat-iron," 1725, verbal noun from iron (v.). Ironing-board attested from 1762.
ironmonger (n.): also iron-monger, "dealer in iron-ware," mid-14c. (mid-12c. as a surname), from iron (n.) + monger (n.). Early forms also include ismongere, irenmanger, iremonger. A street named Ysmongeres lane is attested in London from c. 1215. Related: Ironmongery.
irons (n.): "iron shackles or fetters," mid-14c., plural of iron (n.).
ironside: name given to a man of great hardihood or bravery, c. 1300, first applied to Edmund II, king of England (d.1016), later also to Oliver Cromwell and his troops. Old Ironsides as a nickname of U.S.S. "Constitution" dates from that ship's defeat of H.M.S. "Guerriere" on Aug. 19, 1812, in the War of 1812.
ironstone (n.): 1520s, from iron (n.) + stone (n.). As a type of hard, white pottery, 1825.
ironwork (n.): also iron-work, "objects made of iron," early 15c., from iron (n.) + work (n.). Related: Iron-worker (15c.). Iron works "iron foundry" is from 1580s.
irony (n.): "figure of speech in which the intended meaning is the opposite of the literal meaning" (usually covert sarcasm under a serious or friendly pretense), c. 1500, from Latin ironia, from Greek eironeia "dissimulation, assumed ignorance," from eiron "dissembler," perhaps related to eirein "to speak," from PIE *wer-yo-, suffixed form of root *were- (3) "to speak" (see verb). Used in Greek of affected ignorance, especially that of Socrates, as a method of exposing an antagonist's ignorance by pretending to modestly seek information or instruction from him. Thus sometimes in English in the sense "simulated ignorance."

For nuances of usage, see humor (n.). In early use often ironia. Figurative use for "condition opposite to what might be expected; contradictory circumstances; apparent mockery of natural or expected consequences" is from 1640s, sometimes distinguished as irony of fate or irony of circumstances. Related: Ironist. A verb ironize "speak ironically" is recorded from c. 1600.
irony (adj.): "of or resembling iron," late 14c., from iron (n.) + -y (2).
Iroquois: 1660s (adj.); 1670s (n.) "member of the confederated Indian tribes of central New York," from French (c. 1600); not an Iroquoian word, perhaps from an Algonquian language. Related: Iroquoian (1690s). Originally the Mohawks, Oneidas, Onodagas, Cayugas, and Senecas.
irradiance (n.): 1660s, from Latin irradiantem (nominative irradians), present participle of irradiare "to shine forth" (see irradiate). Related: Irradiancy (1640s).
irradiant (adj.): 1520s, from Latin irradiantem (nominative irradians), present participle of irradiare "to shine forth" (see irradiate). Related: Irradiantly.
irradiate (v.): c. 1600, "to cast beams of light upon," from Latin irradiatus, past participle of irradiare "shine forth, beam upon, illumine," from assimilated form of in- "into, in" (see in- (2)) + radiare "to shine" (see radiate (v.)). Meaning "expose to radiation other than light" (originally X-rays) is from 1901. Related: Irradiated; irradiating.
irradiation (n.): 1580s, in reference to light (literally and figuratively), from French irradiation, noun of action from past participle stem of Latin irradiare (see irradiate). Of X-rays, etc., from 1901.
irradicable (adj.): "that cannot be rooted out," 1728, from assimilated form of in- (1) "not, opposite of" + radicable, from Latin radix "root" (see radish), Latin radicare meant "to take root," and English irradicate (v.) means both "root out" (1709) and "to root, fix by the root" (1660s).
irrational (adj.): late 15c., "not endowed with reason" (of beasts, etc.), from Latin irrationalis/inrationalis "without reason, not rational," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "of or belonging to reason, reasonable" (see rational (adj.)).

Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally. The mathematical sense "inexpressible in ordinary numbers" is from late 14c. in English, from use of the Latin word as a translation of Greek alogon in Euclid.
irrationality (n.): 1560s, originally in the mathematical sense, from irrational + -ity. Meaning "unreasonableness, absurdity" is from 1640s.