Key takeaways
- The US-Ireland and US-UK treaties each cap withholding on portfolio dividends at 15%, so a UK investor gives up the same 15% of US dividends through either domicile.
- On the S&P 500's 1.10% index dividend yield at 6 August 2026, 15% withholding costs about 0.17 percentage points a year — roughly four times the 0.04-point fee gap between the two funds compared here.
- The Irish fund's audited accounts for the year to 31 July 2025 show $204,953,000 of non-reclaimable withholding tax against $1,454,705,000 of dividend income, an effective 14.1%.
- With no valid treaty claim on file the rate is the statutory 30%, which on the same yield is 0.33 points a year — twice the treaty cost, for identical shares.
- US estate tax reaches a non-US investor's US-situated assets above $60,000, against $13,990,000 for a US decedent dying in 2025 — a threshold 233 times smaller.
Both funds hold the same shares, and both hand over 15% of the dividend
You're weighing an Irish-domiciled S&P 500 tracker against a US-domiciled one, and you want to know which loses less to tax. On dividends, for a UK investor, neither does. Both routes give up 15% of US dividend income, because two separate treaties happen to set the same ceiling.
Article 10(2)(b) of the US-UK convention signed in 2001 caps the US tax on portfolio dividends at "15 per cent. of the gross amount of the dividends in all other cases". Article 10(2)(b) of the US-Ireland convention, in force from 1 January 1998, uses almost the same words: "15 percent of the gross amount of the dividends in all other cases". The IRS's own treaty table, revised in May 2023, prints 15 in the dividend column for Ireland and for the United Kingdom alike, against 30 for "Other Countries".
So the popular framing — Irish domicile as a withholding-tax fix — isn't quite right for a UK holder. What actually differs is where in the chain the 15% is taken, who could ever reclaim it, and what happens to the holding when you die. That last one is the part almost nobody prices.
The tax is charged once, but at a different point in the chain
Two layers exist in principle. The first is what the fund suffers on the dividends it receives. The second is what you suffer on what the fund pays out.
Take the Irish route first. A US company pays a dividend to a fund resident in Ireland, and the US withholds at the treaty ceiling. The fund itself pays nothing further at home: iShares VII plc's prospectus states that the company "is not chargeable to Irish tax on its income and gains". And when money reaches you, the prospectus is equally plain — "No tax will arise on the Company in respect of chargeable events in respect of a holder of Shares who is neither Irish Resident nor Irish Ordinary Resident". One layer of tax, taken inside the fund, invisible on your statement.
Now the US route. The dividend is paid to a fund that is itself American, so nothing is withheld on the way in — the withholding rules bite on payments to foreign persons, and a US-domiciled fund isn't one. The tax arrives later, when the fund distributes to you. The IRS table settles that this is the 15% column: its footnote says "The rate in column 6 applies to dividends paid by a regulated investment company", and column 6 for the United Kingdom reads 15.
Same destination, different door. And it's why comparing the two on dividend withholding alone produces a tie.
What 15% costs on a 1.10% dividend yield
State Street's fund page for its S&P 500 trust put the index dividend yield at 1.10% on 6 August 2026 — "the weighted average of the underlyings' indicated annual dividend divided by price". BlackRock's US-domiciled iShares Core S&P 500 ETF reported a 12-month trailing yield of 1.09% at 30 June 2026, which is close enough to corroborate the order of magnitude.
Run 15% across a 1.10% yield and the answer is about 0.17 percentage points of return a year. That's the whole withholding argument, in one number, and it applies to both domiciles.
Worth setting against something. The Irish-domiciled iShares Core S&P 500 UCITS ETF (CSPX) charges a total expense ratio of 0.07%. Its US-domiciled sibling, the iShares Core S&P 500 ETF (IVV), charges 0.03%. The fee gap is 0.04 points. Withholding is roughly four times larger than the fee difference people actually argue about — and it doesn't move when you switch domicile.
One caution on scale. The drag rises and falls with the dividend yield, not with anything you control, so a decade of higher payout ratios would produce a bigger number than 0.17. The figure is a snapshot, not a projection.
The fund's own accounts say 14.1%, and don't explain the gap
Treaty rates are ceilings. What a fund actually suffers is a different question, and for once you can check it.
iShares VII plc's annual report for the financial year ended 31 July 2025 reports dividend income for the Core S&P 500 UCITS ETF of $1,454,705,000. The taxation note for the same fund and the same year shows "Non-reclaimable overseas income withholding tax" of $204,953,000. Divide one by the other and you get 14.1%.
That's a shade under the 15% ceiling, and the accounts don't reconcile the difference. I'm not going to guess at it. What the number does establish is that the withholding is real, audited, and roughly the size the treaty implies — which is more than most cost comparisons can say for the figures they quote. The chart above plots it alongside the treaty rates it sits between.
It also shows why this cost never appears on a statement. It's taken before the money reaches the fund, so it surfaces only as a slightly lower return. That is the same reason it belongs in the wider stack of charges that sits outside a fund's headline fee rather than in the fee comparison itself.
The 30% version is a paperwork failure, not a domicile decision
The treaty rate isn't automatic. Publication 515, issued for use in 2026, states the default: "Most types of U.S. source income received by a foreign person are subject to U.S. tax of 30%." The IRS treaty table says the same thing from the other end, listing 30 for "Other Countries" — meaning everyone without a treaty claim.
Getting from 30 to 15 requires a valid certification of foreign status with your broker. On a 1.10% yield the difference between the two is 0.33 points a year against 0.17 — you'd be paying twice the tax on identical shares, for a form.
Here's the asymmetry that matters. On the Irish route, the treaty claim is the fund's problem and the fund has strong incentives to keep it current. On the US-domiciled route, the claim is yours, made through your platform, and certifications expire. Neither domicile is inherently safer. One of them just moves the failure mode onto your desk.
A pension wrapper rescues a direct US shareholding, not a US fund
This is where the received wisdom breaks, and the treaty text is worth reading slowly.
Article 10(3)(b) of the US-UK convention removes the withholding entirely where the beneficial owner is "a pension scheme, provided that such dividends are not derived from the carrying on of a business, directly or indirectly, by such pension scheme". A SIPP holding US shares directly can reach zero on that wording.
Then Article 10(4) takes it back for funds: "Sub-paragraph a) of paragraph 2 and paragraph 3 of this Article shall not apply in the case of dividends paid by a pooled investment vehicle which is a resident of a Contracting State." And the convention defines a pooled investment vehicle to include an entity "which is entitled to a deduction for dividends paid to its shareholders in computing the amount of its income, profits or gains" — which is exactly how a US regulated investment company is taxed. The IRS table's footnote points the same way, attaching the 15% column rate to RIC dividends.
So a SIPP holding a US-domiciled S&P 500 fund reads 15%, not zero. A SIPP holding the Irish fund also reads 15%, because that treaty benefit belongs to the fund's Irish residence rather than to your wrapper. The pension exemption is real, and a pooled fund of either domicile sits outside it.
That reading is treaty text plus an IRS table, not a ruling on a named fund. A platform or a tax authority could read paragraph 4 differently, and if you're relying on it, that's a question for someone who can see your papers.
US estate tax reaches the US-domiciled fund and stops at the Irish one
Now the part that rarely makes the comparison tables.
The IRS's Internal Revenue Manual states the situs rule in seven words: "Stock of U.S. corporations is considered property in the United States." Shares in a US-domiciled fund are shares in a US entity. Shares in an Irish-domiciled UCITS are shares in an Irish company, and fall outside that rule.
The thresholds are not symmetric. The instructions for Form 706-NA, revised in September 2025, require a return where "the date of death value of the decedent's U.S.-situated assets, together with the gift tax specific exemption and the amount of adjusted taxable gifts, exceeds the filing threshold of $60,000". The IRS adds, on its own guidance page, that this threshold "is not indexed for inflation". The matching credit is small: "In general, the maximum unified credit is $13,000." A US decedent dying in 2025 had a basic exclusion amount of $13,990,000 — a threshold 233 times larger for the same asset.
The rates are the ordinary estate tax rates. Table A of the Form 706 instructions puts the tentative tax on a $500,000 taxable estate at $155,800, and charges 40% on anything above $1,000,000. Take a $500,000 US-situated holding with no deductions: $155,800 of tentative tax, less the $13,000 credit, leaves $142,800. That's 28.6% of the position, payable by an estate that may never have set foot in the United States.
The 1979 treaty removes most of that exposure, and it turns on domicile
The $142,800 is the gross exposure, not the usual outcome. The US-UK estate and gift tax convention, given effect in the UK by an order made in 1979, says in Article 5 that where "the decedent or transferor was domiciled in one of the Contracting States at the time of the death or transfer, property shall not be taxable in the other State", subject to the articles on real property and business property. For a UK-domiciled decedent holding US fund shares, that is the whole ball game. The article doesn't apply if the decedent was a US national.
Two caveats stop this being a clean win. Domicile isn't residence — it's a legal question decided on the facts of a whole life, and a UK resident who isn't UK domiciled can't rely on Article 5. And relief has to be claimed. The Form 706-NA instructions require a statement attached to the return where the position is treaty based, and the IRS issues a transfer certificate only to confirm that "the tax imposed upon the estate, if any, has been fully discharged or provided for", waiving it where an executor is appointed and acting within the United States.
The realistic cost of US domicile here, for most UK-domiciled holders, isn't tax. It's a US federal filing, a wait for a certificate, and an estate that can't easily sell the position until it arrives.
The strongest case for treating all of this as noise
There's a serious objection, and it deserves its best form.
Start with the arithmetic. If withholding is 15% either way, the domicile debate contributes nothing on the largest line, and 0.17 points a year is small next to the year-to-year spread of equity returns. The US fund is cheaper on fees, at 0.03% against 0.07%. The estate exposure is removed by treaty for a UK-domiciled holder. On that reading, the whole question is a tie that got mistaken for a decision.
The objection weakens in three places. The 15% equivalence holds only while the treaty claim holds, and the failure mode differs by domicile. The estate relief holds only while domicile does, which is precisely the thing that changes when someone moves. And the Irish fund's page flags it as ISA-eligible, which is a practical fact about what a UK platform can actually sell you — a comparison that assumes both options are available may be comparing one real fund with one hypothetical one.
There's also a third route this piece doesn't price. A swap-based fund takes its index exposure by contract rather than by holding the shares, which changes the withholding question entirely and introduces a different one. The counterparty limits on synthetic replication are the right place to weigh that trade. Domicile also isn't the only tax label attached to a fund: UK reporting fund status does more to your after-tax return outside a wrapper than the domicile flag does.
What this evidence can't tell you
Treaty rates are ceilings, and a ceiling isn't a measurement. The one measured figure here — 14.1% — is a single fund, a single financial year to 31 July 2025, and a single company's accounts, which don't break the number down.
The 1.10% yield is a snapshot on 6 August 2026, and every drag figure derived from it moves with the yield. None of this is a forecast, and a sample of one year isn't a trend.
The Article 10(4) reading above is mine, from the treaty text and the IRS table, and I found no published guidance applying it to a named fund. The estate tax section describes the rules, not your circumstances: domicile is determined on facts this piece can't see. And nothing here applies to a US citizen or green card holder living in the UK, whose position differs at every step.
One more limit. Every rate quoted is current as at the documents cited — the 2001 and 1998 conventions, the treaty table revised in May 2023, Publication 515 for 2026, and the 2025 revisions of the Form 706 and Form 706-NA instructions. Tax years matter here more than usual.
What would change the conclusion
If either treaty rate moved. The equivalence exists only because two separately negotiated conventions landed on the same 15%. Change one and the domicile question stops being a tie and becomes arithmetic again.
If dividend yields rose materially. At 1.10% the drag is 0.17 points and the cost of a lapsed treaty claim is another 0.17. Both scale directly with the yield, and neither has anything to do with which fund you picked.
If the $60,000 threshold were indexed. It hasn't moved, and the IRS says plainly that it isn't indexed, while the US decedent's exclusion has climbed to $13,990,000. Every year that gap widens, more ordinary holdings cross a line that was drawn for a different era.
The thing to watch isn't the domicile letter on a factsheet. It's whether your treaty certification is current, and whether whoever settles your estate would know a US filing was needed. A holdings record that shows domicile and currency of each position — LedgerTouch keeps one — answers the first question in a glance. The second one is answered by telling someone, which no software does for you.