What is the most tax-efficient way to invest in ETFs?

US does not have a tax treaty with Singapore. So if you were to buy US stocks or ETFs, you will be charge 30% withholding tax on your dividends from US stocks/ETFs. Ireland have tax treaty with US. So Irish domiciled etfs only need to pay 15% instead of 30% for US stocks.

It's not specific to ETFs but rather the country taxes policy.

UK has a agreement with Singapore to waiver the dividend withholding taxes. Hence purchasing from UK you will get the full dividend amount.

Whereas in US, you have to forgo 30% of the dividend amount as tax. Means the dividend you received is just 30% less than the declared. You do not have to personally do tax filing or anything, its all automatic.