Green’s Dictionary of Slang

1. (Irish) an indeterminate measure, approx 0.3 litres (a half-pint) of beer.

1993		(con. 1930s) M. Verdon Shawlies, Echo Boys, the Marsh and the Lanes 60: The women would slip into the snug for a quiet drink. They drank the ‘meejums’, the half-pints.
1994		C. Houlihan Rose of Tralee Souvenir Programme n.p.: It wasn’t a standard measure but it was very popular a generation ago. When you entered a pub you hardly ever called for a medium as your first drink. You called for a pint. When you had finished it, you proferred your glass and said ‘throw a medium into that’. The quantity poured depended upon the person behind the bar [BS].

2. (Irish) the Irish language [SE medium of communication].

1944		M.J.F. Matthews ‘Puss’ ‘Moryah’ in TCD, A College Misc. 9 Mar. n.p.: What, you did your Christmas shopping through the medium? / You were under your O’Dearest in ’16 [BS].
1991		T. Gray Mr Smyllie, Sir 170: A scheme to encourage the use of Irish in the home by payment of an annual bonus of £2 to parents who succeeded in persuading their children to conduct their everyday lives ‘through the medium’, as we used to say .