From my reading of things you are liable to pay full ie investor stamp duty on the property as would have been paid day one... You seem to have gotten the new property relief which means you paid no stamp duty at all..
From Revenue website:
Clawback
A clawback arises if rent is obtained from the letting of the house or apartment within a period of 2 years from the date of the conveyance or transfer, other than under rent a room arrangements. The clawback amounts to the difference between the higher stamp duty rates and the duty paid and it becomes payable on the date that rent is first received from the property.
Since you paid none, you are now liable for the full amount you should have paid. Here's the bad news...it's due 30 days after you first rent the property, so there will be interest and penalties due too....