] I get a definite feeling of irony going on here, in the phrase `what might be called economy in her own private expences'. But what JA is being ironical about, I don't know. Perhaps Elizabeth was doing it very discreetly, so her husband didn't find out?

Possibly, but that "What might be called economy" suggests to me rather that Elizabeth's fund of spending money was so large that all she had to do in order to be "economical" was simply not go spending-crazy. In other words, behave the way she always behaved, just give some of the leftovers to Lydia. I can't really see her hiding it from Darcy, first of all because he probably wouldn't expect her to do any less than help her sister, secondly because when Lydia comes to visit them (while Wickham is off somewhere else "enjoying himself" - ahem -) she'd probably blat it all out anyway, or at least give some very broad hints. "OH! But I promised so faithfully..." No, Lizzy might as well just tell Darcy what she's doing and be done with it.