Reference

When two countries want to tax you.

Cross-border employment, foreign rental income, dual citizenship, retirement abroad — each scenario triggers a different tax-treaty mechanism. The bracket calculator on this site does not handle any of them. Here's why, and what to do instead.

Three core concepts

Residence. The country in which you are tax-resident in a given tax year. Tested by physical presence, centre of vital interests, and other tie-breakers in tax-treaty rules. Most jurisdictions tax residents on worldwide income.
Source. The country where the income arose — where the employment was performed, where the property is located, where the dividend-paying company is based.
Domicile. A common-law concept (UK, Ireland) distinct from residence. Affects how foreign income is treated by the residence country — particularly the “remittance basis” available in Ireland to non-Irish-domiciled residents.

The treaty network

Approximately 3,500 bilateral tax treaties exist worldwide. Most follow the OECD Model Tax Convention or the UN Model with local modifications. The treaties allocate primary taxing rights between the two countries and provide a credit or exemption mechanism to prevent double taxation. Without a treaty, the residence country grants foreign tax credits against worldwide-tax liability; with a treaty, the allocation can be more favourable.

For most cross-border situations, the OECD Model treaty allocates:

Employment income primarily to the country where the work is physically performed (with a 183-day rule for short-term assignments).
Real estate income to the country where the property is located.
Dividends, interest, royalties at reduced withholding rates (typically 10–15 %) to the source country, with the residence country giving credit.
Pensions typically to the residence country, with exceptions for government and military pensions.

Five common scenarios

1. Cross-border worker (live in IE, work in NI)

Both countries claim taxing rights. Treaty rules typically grant primary taxing rights to the country of employment (NI), with the residence country (IE) giving credit for tax paid abroad. Practical effect: you complete two returns, but pay roughly the higher of the two countries' liabilities, not both.

2. Remote worker for foreign employer (resident in IE, employed by US company)

Increasingly common post-2020. Tax depends on whether you create a “permanent establishment” for the employer in your country of residence. If yes, the foreign employer must register and operate payroll locally. If no, you may be subject to self-employment-style tax in your country of residence even though the employer is foreign. The case-by-case analysis is complex; this is one of the highest-risk scenarios for unintended non-compliance.

3. Foreign rental income (UK property owned by Irish resident)

UK has primary taxing rights on UK-source rental income; Ireland gives credit. The Irish resident must report the foreign income on their Irish return and claim a credit for UK tax paid. For Irish-domiciled residents, the income is taxed in full in Ireland; non-Irish-domiciled residents may use the remittance basis.

4. US citizen abroad

Unique among major jurisdictions: the US taxes citizens worldwide regardless of residence. A US citizen living in Ireland files US Form 1040 every year, with foreign earned income exclusion (FEIE) up to $130,000 (2025) and/or foreign tax credits to mitigate double taxation. Compliance burden is severe; engaging a US-qualified preparer is effectively mandatory.

5. Retirement abroad (Irish resident drawing UK pension)

UK private pensions are typically taxed in the country of residence under most modern treaties. UK State Pension is similarly relocatable. UK government and military pensions may be taxed in the UK regardless. The Irish resident reports the foreign pension on Form 11 and pays Irish income tax on it.

What the calculator can and can't do

The calculator computes a single-jurisdiction liability under that jurisdiction's bracket schedule. It does not handle:

Foreign tax credit calculations
Treaty-based allocation of income
Domicile-vs-residence distinctions
The 183-day rule and other physical-presence tests
Permanent establishment analysis for remote workers
FEIE, FTC, totalisation agreements (US-specific)

For cross-border situations, treat the single-jurisdiction figure as a partial input only. Engage a qualified tax adviser in each relevant jurisdiction. The cost (typically €1,000–€3,000 per return) is invariably cheaper than the penalty for getting it wrong.

The minimum viable cross-border action

If you are in a cross-border tax situation:

Confirm your tax residence in each jurisdiction by reference to that jurisdiction's tests.
Identify the relevant tax treaty (most are in force between major economies; verify with a quick OECD treaty search).
Determine which country has primary taxing rights for each category of income you have.
File in both countries; claim treaty credits or use the residence-country foreign tax credit.
For US citizens: engage a US-qualified preparer regardless of where you live.