In Ireland today a "naggin" is a small bottle of liquor in the familiar hip flask shape. (A bottle nearly twice as large, less easily smuggled into events where alcohol is banned, is called a shoulder or daddy naggin.) In Ulysses the word carries the same meaning but can also refer to the liquid measure contained in naggin bottles, or to drinking vessels of that size. It comes from the Irish naigín or noigín (possibly an offshoot of the English "noggin"), which originally named a kind of small wooden pail that served as a drinking cup.

John Hunt 2019

A naggin of Irish whiskey. Source:

Three old Irish noggins, in an image accompanying Claudia Kinmonth's essay. Source: