It is difficult to overestimate just how deliberately and gleefully parochial the writing of Brian O'Nolan can be, whether he was writing as Flann O'Brien or Myles na gCopaleen. Without detracting from his universal appeal, much of its satire cannot be fully appreciated without an understanding of the historical Ireland of his time and before. A working knowledge of the Irish language, French and Latin is often just as necessary. (And yet all without any of the weighty pretension of the joyless obfuscatory chore that is Finnegan's Wake - but I digress.)
Flicking through a few pages of The Third Policeman, here are some of the many things that no Irish person has ever said (which any Irish reader - and many others - would, of course, know):
"by the Holy hokey"
"Holy suffering senators"
"an arm that's as strong as an article of powerful steam machinery"
Note here both the malapropism and the entirely original turn of phrase at the end:
"...a man can have more disease and germination in his gob than you'll
find in a rat's coat"
That's all by way of context, to show that much of the phrasing and speech patterns in The Third Policeman has been cut from whole cloth by Brian O'Nolan and is often both confused and confusing.
To the matter at hand:
"He is worth a packet of potato-meal."
In this instance, I would agree with previous commentators that it is a take on "worth a packet" with added ironic understatement (i.e. Mathers is, indeed, rich) and a touch of some faux Irish flavour. It isn't any real Irish idiom I've ever heard but, rather, par for the odd, convoluted and invented course that is Flann O'Brien.
Whether you wish to attribute it to style, humour for its own sake, or to an effort to skew the reader's sense of reality is a different question - and one you'll be in a better position to judge by the end of the book.
(I do hope you enjoy it - and any incidental study of de Selby or atomics. And, incidentally, the "potato-meal" has nothing to do with chips: potato flour is what is meant.)