Interesting.
Interest was paid gross up to 1986, with the exception of Building Societies who paid a composite rate. My understanding is that the composite rate discharged one's tax liability. I will have to check if one was obliged to return it and I would say tentatively yes.
As regards bank interest credited to accounts prior to 1986, this was always liable at your marginal rate of tax as indeed were credit union dividends and both these would have been under Case III.
The Non Resident Accounts interest did not get returned by the Bank in any return that the Bank or branch of the bank were asked to return by the Revenue. This interest was gross.
The Resident Accounts Interest (the majority) were always subject to taxation at your highest marginal rate.
The Banks could be asked by Revenue either branch by branch by an Inspector of Taxes to make a return to him under s175 of the Income Tax Act 1967 of any distribution paid by way of interest that exceeded I£70. Some customers closed their accounts before year end and er did not get included.
If you wanted to find out just how much interest was actually returned under Case III, the answer is very little. You could ask your TD to put down a parliamentary question, and this will cause consernation in Dublin Castle. In other words, what you have highlighted is in fact a far larger taxation evasion scheme than existed for Non Resident Accounts.