Online Etymology Dictionary

irreclaimable (adj.): 1660s, from assimilated form of in- (1) "not, opposite of" + reclaimable (see reclaim (v.)). Related: Irreclaimably; irreclaimability.
irrecognition (n.): 1820, from assimilated form of in- (1) "not, opposite of" + recognition.
irreconcilable (adj.): 1590s, from assimilated form of in- (1) "not, opposite of" + reconcilable, if that word is so old. Or perhaps from French irréconcilable (16c.). Related: Irreconcilably. As a noun, "one who refuses reconciliation or compromise" (especially in politics), from 1748.
irreconciliation (n.): 1640s, from ir- "not, opposite of" + reconciliation. Irreconcilement in the same sense is from 1737.
irrecoverable (adj.): mid-15c., from Old French irrecovrable (Modern French irrecouvrable), from assimilated form of in- "not, opposite of" (see in- (1)) + recovrable (see recover). In same sense irrecuperable (from Late Latin irrecuperabilis) is from mid-14c. Related: Irrecoverably.
irredeemable (adj.): c. 1600, from assimilated form of in- (1) "not, opposite of" + redeemable. Related: Irredeemably.
Irredentist (n.): 1882, member of Italian political party formed 1878 which demanded the annexation of neighboring regions where a part of the population was Italian-speaking (Trieste, South Tyrol, Nice, Corsica, etc.); from Italian Irredentista, from irredenta (Italia) "unredeemed (Italy)," fem. of irredento, from in- "not, opposite of" (see in- (1)) + redento, from Latin redemptus, past participle of redimere (see redemption). Related: Irredentism.
irreducible (adj.): 1530s, from assimilated form of in- (1) "not, opposite of" + reducible. Related: Irreducibly; irreducibility.
irrefragable (adj.): "that cannot be refuted," literally "incapable of being broken down," 1530s, from French irréfragable (16c.) and directly from Late Latin irrefragabilis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin refragari "to oppose, contest," from re- "back" (see re-) + frag-, base of frangere "to break" (see fraction). Related: irrefragably; irrefragability.
irrefrangible (adj.): 1722, "that cannot be broken or violated," from assimilated form of in- "not, opposite of" (see in- (1)) + refrangible. Related: Irrefrangibly.
irrefutable (adj.): "incapable of being disproved," 1610s, from Late Latin irrefutabilis "irrefutable," from assimilated form of in- "not, opposite of" (see in- (1)) + refutabilis "refutable," from refutare (see refute). Related: Irrefutably; irrefutability.
irregardless (adj.): an erroneous word that, etymologically, means the opposite of what it is used to express; probably a blend of irrespective and regardless, and perhaps inspired by the colloquial use of the double negative as an emphatic. Attested from at least 1870s (e.g. "Portsmouth Times," Portsmouth, Ohio, U.S.A., April 11, 1874: "We supported the six successful candidates for Council in the face of a strong opposition. We were led to do so because we believed every man of them would do his whole duty, irregardless of party, and the columns of this paper for one year has [sic] told what is needed.").
irregular (adj.): late 14c., "not in conformity with Church rules," from Old French irreguler "irregular, incapable, incompetent" (13c., Modern French irrégulier), from Medieval Latin irregularis "not regular," from assimilated form of in- "not, opposite of" (see in- (1)) + Latin regularis "having rules" (see regular (adj.)). General sense of "not conforming to regular rules or principles" is from late 15c. "It expresses the fact of being out of conformity with rule, but implies nothing more with certainty. Yet the word is sometimes used in a sinister sense, as though it were a euphemism for something worse." [Century Dictionary] Meaning "unsymmetrical" is from 1580s. In reference to variable stars, from 1797.
irregular (n.): "one not belonging to a regular body" of any sort, "one not subject to or not conforming with established regulations," 1610s, from irregular (adj.). Main modern sense of "a soldier not of the regular army" is from 1747.
irregularity (n.): early 14c., "violation of Church rules governing admission to clerical office," from Old French irregularité (14c.) and directly from Medieval Latin irregularitas "irregularity," from irregularis "not regular" (see irregular (adj.)). Meaning "that which is irregular" is from late 15c.; sense of "state of non-conformity to rule" is from 1590s; meaning "want of symmetry" is from 1640s.
irrelevance (n.): 1735, from irrelevant + -ance. Earlier in the same sense was irrelevancy (1590s).
irrelevant (adj.): 1680s, from assimilated form of in- (1) "not, opposite of" + relevant. Related: Irrelevantly.
It is worth remembering that irrelevant & relieving are the same word; that, presumably, is irrelevant which does not relieve or assist the problem in hand by throwing any light upon it. [Fowler]
irreligion (n.): 1590s, from French irréligion (16c.) or directly from Late Latin irreligionem (nominative irreligio) "irreligion, impiety," from assimiliated form of in- "not" (see in- (1)) + religio (see religion).
irreligious (adj.): c. 1400, from Late Latin irreligiosus "irreligious, impious," from assimilated form of in- "not, opposite of" (see in- (1)) + religiosus (see religious). Related: Irreligiously.
irremediable (adj.): "beyond remedy," mid-15c., from Late Latin irremediabilis "incurable," from assimilated form of in- "not, opposite of" (see in- (1)) + remediabilis "that may be healed, curable" (see remediable).
irremediably (adv.): mid-15c., irremediabili, from irremediable + -ly (2).
irremovable (adj.): "not capable of or subject to removal," 1590s, from assimilated form of in- (1) "not, opposite of" + removable. Related: Irremovably; irremovability.
irreparable (adj.): early 15c., from Old French irréparable (12c.), from Latin irreparabilis "not to be repaired or recovered," from assimilated form of in- "not, opposite of" (see in- (1)) + reparabilis "that can be repaired" (see repair (v.)). Irrepairable, from the English verb, was used 16c.-17c. but seldom was seen after.
irreparably (adv.): mid-15c., from irreparable + -ly (2).
irreplaceable (adj.): 1806, from assimilated form of in- (1) "not, opposite of" + replaceable. Related: Irreplaceably.
irreprehensible (adj.): "blameless," late 14c., from Late Latin irreprehensibilis, from Latin irreprehensus "blameless, without blame," from assimilated form of in- "not, opposite of" (see in- (1)) + reprehensibilis, from past participle stem of Latin reprehendere "to blame, censure, rebuke; seize, restrain" (see reprehend).
irrepressible (adj.): 1767, from assimilated form of in- (1) "not, opposite of" + repressible (see repress).
Increase of population, which is filling the States out to their very borders, together with a new and extended network of railroads and other avenues, and an internal commerce which daily becomes more intimate, is rapidly bringing the States into a higher and more perfect social unity or consolidation. Thus, these antagonistic systems are continually coming into closer contact, and collision results.

Shall I tell you what this collision means? They who think that it is accidental, unnecessary, the work of interested or fanatical agitators, and therefor ephemeral, mistake the case altogether. It is an irrepressible conflict between opposing and enduring forces, and it means that the United States must and will, sooner or later, become either entirely a slaveholding nation, or entirely a free-labor nation. [William H. Seward, speech at Rochester, N.Y., Oct. 2, 1858]
Related: Irrepressibly.
irreproachable (adj.): 1630s, from French irréprochable (15c.), from assimilated form of in- "not, opposite of" (see in- (1)) + réprochable (see reproach (n.)). Related: Irreproachably.
irreption (n.): "a creeping in," 1590s, from Late Latin irreptionem (nominative irreptio), noun of action from past participle stem of Latin irrepere, from assimilated form of in- "in" (see in- (2)) + repere "to creep" (see reptile).
irreputable (adj.): "disreputable," 1709, from assimilated form of in- (1) "not, opposite of" + reputable.
irresistance (n.): 1640s, from assimilated form of in- "not, opposite of" (see in- (1)) + resistance.
irresistible (adj.): 1590s, from Late Latin irresistibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + resistere (see resist). Related: Irresistibly; irresistibility.
irresolute (adj.): "not firm in purpose, wavering, given to doubt or hesitation," 1570s, from Latin irresolutus "not loosed, not loosened," from assimilated form of in- "not, opposite of" (see in- (1)) + resolutus (see resolute). Related: Irresolutely.
irresolution (n.): 1590s, from assimilated form of in- (1) "not, opposite of" + resolution. Perhaps from or based on French irrésolution (16c.).
irresolvable (adj.): 1650s, "insoluble," from assimilated form of in- (1) "not, opposite of" + resolvable. Meaning "that cannot be resolved into parts" is from 1785. Related: Irresolvably.
irrespective (adj.): 1620s (implied in irrespectively), "disrespectful," from assimilated form of in- (1) "not, opposite of" + respective in its sense of "regardful." Meaning "without taking account of particular circumstances or conditions" had developed by 1690s, from the notion of "not observing or noting with attention." In modern use it tends to be adverbial, in irrespective of, a use attested by c. 1800.
irresponsibility (n.): 1767; see irresponsible + -ity.
irresponsible (adj.): 1640s, "not legally answerable for conduct or actions," from assimilated form of in- (1) "not, opposite of" + responsible. Meaning "not acting with a sense of responsibility" is from 1680s. Related: Irresponsibly.
irresponsive (adj.): "not responsive, not answering," 1797, from assimilated form of in- "not, opposite of" + responsive.
irretractable (adj.): 1744, from assimilated form of in- "not, opposite of" + retractable.
irretrievable (adj.): "not recoverable," 1690s (implied in irretrievably), from assimilated form of in- (1) "not, opposite of" + retrievable. Related: Irretrievability.
irreverence (n.): mid-14c., from Old French irreverence (13c.) or directly from Latin irreverentia "want of reverence, disrespect," from irreverentem (nominative irreverens) "disrespectful, irreverent," from assimilated form of in- "not, opposite of" (see in- (1)) + reverens, present participle of revereri "to stand in awe of" (see revere).
irreverent (adj.): "deficient in veneration or respect," mid-15c., from Old French irreverent or directly from Latin irreverentem "disrespectful, irreverent" (see irreverence). Related: Irreverently (early 15c.); irreverential. Irreverend (late 15c.) means "not worthy of respect or veneration."
irreversible (adj.): 1620s, of decrees, etc., "that cannot be overturned or undone," from assimilated form of in- (1) "not, opposite of" + reversible. Of physical things, "that cannot be turned the other way," from 1821. Related: Irreversibly.
irrevocable (adj.): also irrevokable, late 14c., from Latin irrevocabilis "that cannot be recalled, unalterable," from assimilated form of in- "not, opposite of" (see in- (1)) + revocabilis (see revoke). Related: Irrevocably.
irrigable (adj.): 1813, from Latin stem of irrigate (v.) + -able.
irrigate (v.): "supply land with water," 1610s, from Latin irrigatus, past participle of irrigare "lead water to, refresh, irrigate, flood," from assimilated form of in- "into, in, on, upon" (see in- (2)) + rigare "to water, to moisten," of uncertain origin. Perhaps [Watkins] from PIE *reg- (2) "moist" (see rain (n.)). De Vaan offers as possibilities the root of regere "to direct, lead," on the notion of leading water onto the fields, or to the root of rigere "be stiff," literally "stretch." The first better suits the sense, but has phonetic problems.

Related: Irrigated; irrigating. In Middle English it was an adjective, "watered, flooded" (mid-15c.). Other adjectival forms have been irriguous (1650s), irrigative (1842), irrigatorial (1867).
irrigation (n.): 1610s, "a supplying of water to land," also in medical use, "supply of a liquid to some part of the body," from Latin irrigationem (nominative irrigatio) "a watering, irrigation," noun of action from past participle stem of irrigare "lead water to, irrigate, flood" (see irrigate).
irrisory (adj.): "given to sneering or laughing derisively at others," 1824, from Late Latin irrisorius "mocking," from irrisor "a mocker," from stem of Latin irridere "to laugh at, make fun of," from assimilated form of in- "in" (see in- (2)) + ridere "to laugh" (see risible). Related: irrision (1520s), from Latin irrisionem, noun of action from the verb.
irritability (n.): 1755, from irritable + -ity or else from Latin irribilitas.