I will be making a payment to Revenue next week for a gain in period 1 (to November 30th), however I anticipate recognising losses in period 2 (December 1st to December 31st). At what point does this get netted off and my refund issue?
It will get netted whenever you make a tax return on any of Forms 11, 12, or CGT1. However, if you are certain you are going to net out to an amount less than the CGT exemption limit of €1,270, don't bother making a payment for period 1 at all. Likewise if you have a very good idea what your net liability is going to be, make a payment for that amount or a little above. But don't get it wrong or you will be hit for interest.
Going by their exact wording, any of the three tax return forms will only ask you for details "In respect of net chargeable gains that arose in the period" for each of the two periods. The section preceding the questions about the individual periods makes it clear that the net chargeable gain is the amount chargeable for the full year. So regardless of how much your gain is, you will never be filling in an amount higher than your annual net gain for period 1. As long as you can anticipate what that is, I therefore see no reason to make a payment higher than that amount for period 1. Was discussed here on AAM before, and I've followed this approach myself.