If you make a loss then you can carry that loss forward indefinitely and set it against future capital gains. When you do make a capital gain you MUST offset the previously incurred loss first (e.g. before your annual CGT allowance etc.). I think that you are supposed to make a CGT return even if you make a loss in order to "record" the loss with Revenue but I'm not 100% sure on this and I have previously offset losses that were not recorded in this way in the past without any problems...