As I understand it, EU rules mean that interest earned in an EU country is taxed at the same rate as in Ireland ie DIRT (this is to ensure that the single market is a reality) and no taxes should be deducted as per double taxation treaties with EU countries
If it is a non-EU country, then the rights to tax the interest is laid out in the double taxation treaty. In Ireland, it will probably be taxed at your marginal tax rate
Normally you will declare the gross amount as EU earned interest or non-EU earned interest along with the foreign tax deducted, if any