The foreign tax on PAC is a tax would be paid to the Mexican government. The Mexican government doesn’t care what type of American account PAC is held in - they just want to get the tax that is owed on the dividends. Neither you nor your IRA will be able to claim a tax credit on the foreign tax paid.
From a quick search, it appears that Mexico levies a 10% withholding on dividends paid to US accounts, so you should discount that dividend by 10% when determining whether it’s a good investment for your IRA. You should confirm the withholding yourself before making a purchase.
AJ