Green’s Dictionary of Slang

1. (Irish) Guinness stout, seen as the basic Irish drink.

1939		‘Flann O’Brien’ At Swim-Two-Birds 109: In time of trouble and lousy strife / You have still got a darlint plan, / You still can turn to a brighter life – / A PINT OF PLAIN IS YOUR ONLY MAN!
1946		D. MacDonagh Happy as Larry Act IV: As weak as the watery drops of bad plain.
1989		H. Leonard Out After Dark 194: It was enough to pay for two pints of ‘plain’.
1994		(con. 1930s) K.C. Kearns Dublin Tenement Life 96: She had a begging can with a lid on it and she’d say, ‘Go down to Mr Quinn and tell him to give you a nice creamy gill of plain’.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Pint of plain (n): a pint of Guinness.

2. (drugs) see spice n.³