Vanguard UK lets you invest in the S&P 500 index fund via an ISA (Independent Savings Fund). I understand that an ISA in the UK shields you from any tax on your profits from investments (up to a limit). This fund is authorised in Ireland and regulated by the Central Bank of Ireland.
I'm confused. There is no mention of paying a US tax. As a UK citizen, would I have to pay a tax on my profits from investing in US companies? Do I have to sort this out independently with the IRS or do I sort that out with HMRC? Thanks in advance.