I've never heard of anyone recieving this sort of refund if not from the employer (in the next pay period) or directly from Revenue (usually takes a lot longer than a single month to process). I am however, as always, open to correction if someone knows of a side door I've yet to come across.
From a practical point of view, I'd aim to sort out alternative solutions so you don't end up pulling your hair out. Not sure of your exact situation, but have a quick search through a few of the threads here, find a loan/overdraft/etc that will give you enough to cover the mortage in the short term. You'll be in a position to pay it off in full next month when the refund is given so as long as it has a slight interest free period and no penalties for paying off early it shouldn't cost you an arm and a leg.... Worth organising for the piece of mind alone.