Green’s Dictionary of Slang

1. (Scot./Irish) alcohol, a drink.

1996		J. Murphy A Picture of Paradise in McGuinness Dazzling Dark (1996) Act II: Second swally and the belly starts to warm up ... you’re on fire! Third swally and that’s it, you’re away!
2018		I. Welsh Dead Man’s Trousers [37]: Who’s for champers? A wee swally ay Bolly?

2. a drinking binge.

1996		A. Close Official and Doubtful 237: Have to organise a swally to see you off.
1999		Indep. 1 Nov. 7: Many of the words in his patter have become part of the national language [...] boak, which means sick; and swally, a booze-up.
2020		G. Armstrong Young Team 5: It’s [i.e. Halloween] a good excuse for a swally.