Green’s Dictionary of Slang

(Irish) methylated spirits, as drunk by alcoholics; thus chat-shop, a place where one drinks methylated spirits.

1939		Joyce Finnegans Wake (1959) 43: A half sir from the weaver’s almshouse who clings and clings and chatchatchat clings to her.
1953		B. Behan Scarperer (1966) 11: They were in some other chat-shop. [Ibid.] 211: He slept on the landings of tenement houses and spent the price of his flop, when he had it, on chat.
1956		B. Behan Quare Fellow (1960) Act I: There was one day I was brought in for drinking the chat and I went to court that morning.