26 CFR 513.2: Dividends.

The fact that the payee of the dividend is not required to pay Irish tax on such dividend because of the application of reliefs or exemptions under Irish revenue laws does not prevent the application of the reduction in rate of United States tax with respect to such dividend. If the dividend would have been subject to Irish tax had the payee thereof derived an income large enough to require payment of tax then liability to Irish tax exists for the purpose of the reduction in rate of United States tax. As to what constitutes a permanent establishment, see Article II(1)(i) of the convention.