Following John Bogle advice, I would like to invest in S&P 500 regularly, about $1000 every month. Which platform should I use to buy Ireland-domiciled S&P 500 ETF?

Any platform that allows access to the London Stock Exchange (LSE) can be used. Have to do your due diligence on the costs between the platforms.

Any stock dividends from US companies and paying to non-US persons are subject to withholding tax(usually 30%).

In the iShares Core S&P 500 UCITS ETF (Acc), the dividends are subjected to withholding tax of 15% rather than 30% due to the tax treaty between USA and Ireland.

But as it is a accumulating ETF, the dividends net of withholding tax is not paid out but reinvested back into the ETF automatically.

For SG, not a lot of robo-advisors offer Ireland-domiciled ETFs, last I checked. You may need to DIY.