My understanding is that you can only claim vat from the time of registration. So lets say you set up a company in today, go and buy furniture, office supplies etc and register for vat next month, well its only from next month that you can claim vat back.
I think it was mentioned on AAM in the past that you can ask at registeration to allow backdating of invoices and if you can give a reasonable answer then revenue may allow you to do this. However if you have issued invoices to your customers you will have to reissue the invoices and collect vat and forward to Revenue (not a great way to start a business relationship).