Is it ensure, insure, or assure?
c.1200, Irisce, from stem of Old English Iras "inhabitant of Ireland," from Old Norse irar, ultimately from Old Irish Eriu (accusative Eirinn, Erinn) "Erin," which is from Old Celtic *Iveriu (accusative *Iverionem, ablative *Iverione), perhaps from PIE *pi-wer- "fertile," literally "fat," from root *peie- "to be fat, swell" (see fat (adj.)).
Meaning "temper, passion" is 1834, American English (first attested in writings of Davy Crockett), from the legendary pugnacity of Irish people. Irish-American is from 1832; Irish coffee is from 1950. Wild Irish (late 14c.) originally were those not under English rule; Black Irish in reference to those of Mediterranean appearance is from 1888.