First thing is first - the system is self assessment so it's up to you to claim relief for any losses you have carrying forward.
However you must also be able to stand over these losses, in the event that Revenue query or audit your current returns.
If you have been filing tax returns all along you could / should have been entering the amount of the CGT losses you had carrying forward, that way if you make a gain and use the losses in a later year, when Revenue look at it they'll see you were carrying the losses forward for several years...
That's not to say that there'll necessarily be any difficulty with the issue when you do come to make the gain and use the losses, but you'll see how from Revenue's perspective you suddenly and conveniently having a loss that they never knew about previously could give rise to a question.