cork, n.⁴ — Green’s Dictionary of Slang

cork n.⁴

[proper name Cork, a city from whence many immigrants arrived in the US]

(US) an Irish person; thus corktown, cork hill, cork hollow, the Irish part of a town or city.

1950		WELS n.p.: (Common nicknames for people living in nearby settlements or places) Cork Hollow (the Irish).
1966–68		in DARE n.p.: (Names and nicknames around here for people of foreign background...Irish) Corks; (Nicknames for the different parts of your town or city) Corktown; (Names or nicknames for the part of a town where foreign groups live) Cork Hill: the Irish section.