The wording of the relevant Article in the Swiss DTT is:
"Subject to the provisions of the law of Ireland regarding the allowance as a credit against Irish tax of tax payable in a territory outside Ireland (which shall not affect the general principle hereof):
Swiss tax payable under the laws of Switzerland and in accordance with this Convention, whether directly or by deduction, on profits, income or chargeable gains from sources within Switzerland (excluding in the case of a dividend tax payable in respect of the profits out of which the dividend is paid) shall be allowed as a credit against any Irish tax computed by reference to the same profits, income or chargeable gains by reference to which the Swiss tax is computed..."
The Case III income is correctly chargeable for USC since it is part of Total Income. There is no provision to confer a credit against USC for the foreign tax paid, and if you think about it, would you take the credit against income tax first, and then against USC, or vice versa, or would the credit be available against both, in the absence of a provision to state otherwise... I'd suspect the Revenue interpretation of the DTT would be that USC is by definition not income tax, nor would it fall within the definition (per Article 2 of the DTT) of "identical or substantially similar taxes which are subsequently imposed in addition to, or in place of, the existing taxes".
Schedule 24 to TCA 97 states "the Irish taxes means income tax, income levy, universal social charge and corporation tax". It then goes on to say that "Subject to this Schedule, where under the arrangements credit is to be allowed against any of the Irish taxes chargeable in respect of any income, the amount of the Irish taxes so chargeable shall be reduced by the amount of the credit.... Nothing in this paragraph shall authorise the allowance of credit against any Irish tax against which credit is not allowable under the arrangements."
It's an issue I've been looking at recently for an Irish resident relative who is being subjected to the 10% rate of USC on foreign employment income (when he's already paying Income tax at a very high rate in the foreign country), so I'll revert back, probably in a couple of months after some correspondence with Revenue, with something a bit more concrete. Wouldn't be holding out too much hope though, short of someone going to ECJ...