You only have to pay DIRT on deposits, no income tax.
You have to include the gross interest received in your income tax returns if the revenue require you to make a return. They add it to the portion of your income taxed at the standard rate (also 20%) when they're calculating your tax liability. So the net result is you don't pay any more tax on it than the DIRT that's taken at source by the bank on behalf of the revenue.
Qualifying people [broken link removed]