Yes, but I think the rate should be 15% for Irish funds. 30% was what I found for some GB funds: compare dividend yield of VOO and VUSD. As it’s the fund who got hit, not you, you don’t see this withholding and can’t declare it to reclaim on your tax devlaration. You can only claim back what was withheld on a distribution directly to you.
Assuming let’s say 30% swiss marginal income tax rate (your mileage may vary!) and 15% withholding at the fund level, every $1 of original dividends turns into 0.7*0.85 = 0.585 after tax $s in your pocket.
But if 15% are withheld directly on distribution to you, you’ll get $0.85 paid right away, and after taxes this becomes $0.70 (you pay $0.30 tax but get $0.15 back). Or if you bank with a swiss broker, they’ll take 15% + 15% additional swiss withholding = 30% right away, both you can fully offset against your swiss taxes, so net it’s still $0.70 after taxes, but you lose a bit on not being able to reinvest 15% right away until taxes are due
Difference: $0.125 per each $1 of original dividends
How to minimize losses? Use US domiciled funds (no withholding at fund level for US stocks) with a qualified intermediary US broker (15% withholding for CH), such as IB (lowest fees)