As far as I know, a withholding tax of 25% is levied in Ireland, of which 0% is credited in Germany. Because you have already used up your tax-free amount, in addition to the 25% withholding tax in Ireland, the full capital gains tax + solidarity surcharge + any corporation tax will be charged in Germany.
Therefore (without having done the math) I think the credit you have been credited can be correct. You are welcome to do the math.
@Murmel Yes, Ireland is a bit stupid in terms of dividends. If the custodian bank were to pass on the information that you are an EU citizen, your withholding tax in Ireland would be 0%. Unfortunately, only very few banks do this.