@lichtoefur I would suggest to think in terms of risk when it comes to investment. The "downsides" usually happen when we handle the risk sub-optimally without a proper strategy. Downside is absolute whereas risk is relative.
If you are into passive investing, you choose an index fund because the
risk is higher than most bonds but lesser than individual stock-picking. This moderate level of risk may result in a 20% - 30% loss when the market crashes. However, you could mitigate the risk by investing long-term. For a short-term investor, this could be a liquidity problem, and thus a "downside".
You pick Irish-domiciled funds over US-domiciled funds, because as non-US investors, you should navigate US tax traps including estate-tax and withholding tax on dividends. In my opinion, the withholding tax on dividends is a cost issue because if you were to invest long-term, then you must minimize the cost. The US estate tax, however, is a serious risk issue if you care about inheritance.