As long as the losses arose from selling shares for less than they were acquired for (and are not, for example, "paper" losses) then yes. Any capital loss can be carried forward indefinitely and set against subsequent capital gains when calculating one's CGT liabilities.
The company I had the shares in went under (mobile phone company) so I lost the lot. It was a U.K. company and I bought the shares in sterling, would I need to find out what the exchange rate was at the time and convert this to Euros and then re-index the Euro amount to todays value?
Yes - you will need to convert the loss to € using the rate applicable at the time or some other rate that Revenue will accept. If in doubt contact them to get the applicable rate from them. No - you cannot use indexation to enhance a loss so indexation is irrelevant in this case (at least for your loss whatever about your recent gain against which you are offsetting it).