By law, Revenue are not required to pay interest on VAT repayments until after 93 days from receiving a valid claim (i.e. not outstanding information requests etc.) After that the rate is 0.011% per day, so significantly less that the rate you would pay for late payment of tax.
In relation to the OP's post, receiving a VAT repayment in a few days would in my experience be an exception rather than the norm. A few weeks is more standard.
First thing I would do is call the Collector General who should be able to tell you where it is at - has it been referred to your Inspector or is the payment being processed? If it is with the Inspector, give them a call to ask what info they require - will usually be copy invoices - and send this in to them. No point in waiting for their letter in the post if you want the refund.
If you are constantly in a repayment position, you might think about filing your returns just after the period ends - don't wait til the 19th. You can also apply to file monthly VAT returns, but this obviously would not be beneficial if you expect to be VAT payable in the future.