4 years I believe. Provided they are under the tax exemption limit they can submit back claims for 2004-2007 on form [broken link removed]( goes to Revenue) & get the accounts exempt from DIRT from 2008 now on by filing form [broken link removed] ( goes to the relevant bank ) for each account invlolved.
4 years is the limit for any tax claim. You might want to check out the exemption limits for each year, as the DIRT tax is only refundable if their combined income from all sources is below or marginally above the specified exemption limit I think it was roughly 38,000 in 2007 for a married couple over 65.