Green’s Dictionary of Slang

Fenian n.

also Irish cold, three cold Irish
[a pun on ‘three cold Irish’, itself referring to the hanging of the Fenians Allen, Larkin and O’Brien for the ‘Manchester Murder’ of PC Brett in 1867 or for three men, also Fenians, hanged for the Phoenix Park murders of Lord Frederick Cavendish and Thomas Henry Burke, Under-Secretary for Ireland, on 6 May 1882]

[late 19C] threepenny-worth of whisky and water; thus generic for any quantity of whisky.