Green’s Dictionary of Slang

glasheen n.

[SE glass + dimin. sfx -een]

(Irish) a small glass of strong drink.

[Ire]‘Myles na gCopaleen’ Faustus Kelly in ‘Flann O’Brien’ Stories & Plays (1973) 124: ‘Come into the bar,’ says he to me, ‘and have a glasheen.’.