These ROOT-WORDS are the Prefixes IL and its partner IR, meaning NOT. It seems that the Prefix IN is also assimilated with certain consonants. Before a root word beginning with I, IN becomes IL. Before 4, IN becomes IR. All this is to make it pleasant to say and pleasant to hear.
1. Illegal: IL legal (il ee’ gal) adj.
Not according to law
2. Illegible: II legible (il lej’ I b’l) adj.
Not able to be read
3. Illiterate: IL literate (il it’ e rat) adj.
Having little education; cannot read or write
4. Illiteracy: IL literacy (il it’ e ra see) n.
The state of being uneducated; inability to read or write
5. Illusion: IL lusion (il ue’ zhun) n.
An unreal image
6. Illogical: IL logical (il oj’ I kal) adj.
Not having sense
7. Illiberal: IL liberal (il ib’ e ral) adj.
Not open minded; being a bigot
8. Illicit: IL licit (il is’ it) adj.
Not allowed; not lawful
9. Irreclaimable: IR reclaimable (ir e klay’ ma b’l) adj.
Cannot be claimed again
10. Irregular: IR regular (ir eg’ yu lar) adj.
Not as it should be
11. Irreligious: IR religious (ir e lij’ us) adj.
Lacking religious feelings
12. Irreverent: IR reverent (ir ev’ rent) adj.
Showing a lack of respect about holy matters
13. Irreparable: IR reparable (ir ep’ e ra b’l) adj.
Cannot be repaired
14. Irresolute: IR resolute (ir ep’ e ra b’l) adj.
Cannot make up his mind
15. Irreproachable: IR reproachable (ir e proche’ a b’l) adj.
Cannot be blamed in any way
16. Irresponsible: IR responsible (ir e spon’ si b’l) adj.
Not having a sense of duty
17. Irrevocable: IR revocable (ir ev’ o ka b’l) adj.
Cannot be recalled
18. Irresistible: IR resistible (ir e zis’ ti b’l) adj.
Not able to be withstood
19. Irrational: IR rational (ir ash’ us nal) adj.
20. Irrelevant: IR relevant (ir el’ e vant) adj.
Has nothing to do with the case; unrelated
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