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]

threepenny-worth of whisky and water; thus generic for any quantity of whisky.

[UK] ‘’Arry on [...] the Glorious Twelfth’ in Punch 30 Aug. 97/2: Them Moors is the spots for cold Irish, and gives you the primest of pecks.
[UK]W. Pett Ridge Minor Dialogues 205: Will you kindly gi’ me ’n Irish cold.
[UK]J. Ware Passing Eng. of the Victorian Era 129/1: Fenian (Peoples’, 1882 on). Three cold Irish, i.e., threepence worth of Irish whisky and cold water.