Yes, Irish ETFs can’t beat US ETFs in lower total costs of investment (TER + withholding tax on dividends received by the fund + your tax on dividends distributed by the fund). So if you feel fine investing via US ETFs, just go with them. However, stocks index funds in 3a sheltered account (I have finpension) on non-US stocks are more tax efficient than US ETFs. I did some estimations here.
Numbers have changed, I did recalculate few things for myself recently, but the qualitative results have not.
For your other questions you can find lots of useful information if you search this forum.