Green’s Dictionary of Slang

shamrock tea n.

weak tea.

1934		Larne Times 2 June 6/5: ‘Everything all right, sir?’ inquired the land-lady. ‘Yes, thanks; and this shamrock tea is fine!’ ‘Sharock tea! Whatever’s that?’ she asked. ‘Three leaves to the pot’.
1935		Sport (Adelaide) 21 Feb. 1/1: ‘Shamrock Tea’ [...] ‘The worst part of the grub is the “shamrock tea.”’ [...] ‘What's that?’ [...] ‘Three leaves of tea and the pot filled with hot water’.
1937		Times (Shreveport, LA) 5 Dec. 67/1: Why is tea made by a very stingy housewife called ‘shamrock tea?’.
1941		Longreach Reader (Qld) 22 Nov. 21/3: ‘What’s on earth’s shamrock tea?’ ‘You know the kind — made with three leaves’.
1942		Times (Shreveport, LA) 6 Mar. 12/6: Eire’s tea shortage has produced a joker’s shamrock tea, made with three leaves.
1957		Morn. Call (Allentown, PA) 23 Sept. 6/7: Eric ordered ‘string white tea [...] And I don’t mean shamrock tea’.
1979		Toby: A Bristol Tramp Tells His Story in DSUE (1984).
1986		Asbury Park Press (NJ) 17 Mar. 20/2: Shamrock tea wouldn’t be their style now.
1997		Share Slanguage.