'I doubt it' says Croker' is a handy non-committed-but-negative expression, kind of akin to saying: "maybe... but I wouldn't hold me breath".
I love it. I've never heard it outside of Limerick. I haven't heard it often inside of Limerick, come to think of it, but when I do it's always a pleasure.
The origin* of the phrase is that it was the 'famous last words' of a local libertine landlord before he shuffled off his mortal coil.
I'll explain this bit of Limerick lore,(as I heard it,) with, appropriately enough, a Limerick:
A priest went to Croker, and spoke,
at the side of his deathbed; I quote:
"you are going with grace,
to a far better place."
"Oh, I doubt it" said Croker, and croaked.
* Oh yes, it's possibly apocryphal, but who gives a damn about that?