Erin

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.