I don't think that there is a limit, but there are strict rules.
You must elect to use the loss in the actual tax year. So if you have a loss from many years ago and you want to claim it now for 2005, you can't. You have to send a letter now to the Revenue electing to claim it for this year.
When you elect to use your loss, you must use the loss first before the annual exemption.
Brendan