No, its not Property Tax its a Capital Gains Certificate. I finally managed to track down the relevant office in the Revenue and they explained that the Application form (CG50) is submitted to the Revenue before the sale closes, with a copy of the Contract for sale, and a Certificate is issued in about five days. No payment is due or requested at this point, it is pretty much a formality but it has to be done if the value of the property is over €500,000. Once the sale closes, the CGT is calculated and paid by the end of the tax year.
Thanks for your help everyone.