When you submit your CGT return, you just the total of the losses you are claiming and they will be deducted from the gross gain to get a net taxable gain
But make sure you have documentation to back up your figures if Revenue ask for proof of the losses.
You should start by listing your gains and losses for each year, to see if any of your losses are already used up by subsequent gains, or if some of your gains are used up by the €1,270 annual exemption.