Green’s Dictionary of Slang

(Ulster) anything exceptionally good.

1993		(con. 1945) S. McAughtry Touch and Go 57: John has a hell of a peeper [...] A real wheeker.
1996		advert for Northern Ireland Tourist Board in Irish Times 30 May n.p.: Why not consider taking a Belfast City Break [...] you’ll have a ‘wheeker’ of a time [BS].