Green’s Dictionary of Slang

(Irish) an expression of disbelief, a dismissive retort.

1918		Joyce Exiles Act I: beatrice: Did he practice the piano when I was away? brigid: [Laughs heartily] Practice, how are you!
1987		V. Caprani Vulgar Verse and Variations n.p.: So there you are – three generations of double marriages of sisters celebrated. Guinness Book of Records, how are yeh! [BS].
1991		B. Quinn Smokey Hollow 58: Mr Toner put a half-crown on the table and invited them to contribute some of their pocket money to the cause. Pocket money how are ye!