A lot of the ETF's listed in London are domiciled in Ireland, as a few on other European exchanges.
To get the reduced withholding rate in Ireland, depending on where you are from and how the investments are held, can be as simple as a form, or may require a certificate of residence from your country's tax authority.
That being said, I haven't dealt with Irish stocks for a few years through a role change at work, and things may have changed.
