Green’s Dictionary of Slang

(Irish) a portion of fish and chips.

c.1914		C. Mac Garvey Green Line and the Little Yellow Road in Mac Thomáis (1982) 159: Lingering ever near a pub, they would seldom dream of grub, / Save their nightly table d’hote of ‘wan and wan’.
1953		B. Behan Scarperer (1966) 45: I got the two one-and-ones and a bit of salt an’ vinegar.
1961		(con. 1930s) D. Behan Teems of Times and Happy Returns 107: Tonight the long procession for the fish and chip shop, ‘Two one and ones, Mister Savallo, and plenty of salt and vinegar’.
1987		V. Caprani Vulgar Verse and Variations n.p.: Giuseppe Cervi arrived in Dublin in the 1880s [...] Signora Cervi, with very little English at her command, helped behind the counter and generally processed the take-away orders by pointing at the selection of fried fish – ‘uno di questo, uno di quello’ (one of this, one of that). This was soon shortened to ‘uno e uno’, then ‘one and a one’, and in less than a decade Dubliners in general were ordering their portion of chips and one fish as ‘wan and wan’ [BS].
1995		Irish Times 7 Oct. n.p.: ‘One and One’ in Tahiti. The chips are down. The fish are fried to a crisp [by French nuclear testing] .
2001		G. Coughlan Everyday Eng. and Sl. 🌐 One and One (n): fish and chips i.e. One and One Cod.