Why is the ninth month called September?
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.
[dander 1831+; Irish 1834+; origin of dander form unknown; the form rise my dandee up is found in a nautical context in 1839 and is probably related]