Every autumn, the phone rings with a version of the same conversation.
Someone who lives in Newry, or Warrenpoint, or Derry, has been working across the border for a few years. Good job, Irish payslip, tax coming out every month like clockwork. And then a letter arrives from HMRC, or a colleague says something in the canteen, and suddenly there’s a question hanging in the air:
Am I supposed to be doing a UK tax return?
Short answer: almost certainly, yes. The longer answer is below, along with a few important considerations.
The two rules that explain everything
Cross-border tax has a reputation for being complicated. And in fairness, parts of it are — I’ve spent a fair chunk of my career on the fiddly bits. But the framework sitting under it all comes down to two rules, and once you’ve got them, everything else clicks into place.
Rule one: you pay tax where you earn your income. Work in Dundalk, and Irish tax comes off your wages in Dundalk. That’s the PAYE and USC you see on your payslip every month, and it’s exactly as it should be.
Rule two: you declare your worldwide income wherever you’re tax resident — and claim relief for tax you’ve already paid elsewhere.
For the most part, if your only or main home is in Northern Ireland, then in HMRC’s eyes you’re UK tax resident. It doesn’t matter that your employer is in the Republic. It doesn’t matter that you’ve never earned a penny in sterling. Where you live generally decides where you declare — and you live in the UK.
(A footnote for the technically curious: residence has formal day-counting tests, and the two countries don’t even count days the same way. The UK asks whether you were present at midnight; Ireland counts a day if you were there at any point in it. Spend your weeks working in Dundalk and your nights in Newry and you can tick boxes on both sides — it’s surprisingly easy to be tax resident in both countries at once. When that happens, the double taxation agreement breaks the tie, and it largely comes down to where your life is actually centred — your home, your family, your economic interests. For most cross-border workers, that’s the North. But if your situation straddles the line — a place in Dublin during the week, say — it’s worth a proper look rather than a guess.)
So your Irish salary goes on a UK Self Assessment tax return. Every year.
“Hang on — so I’m taxed twice?”
No, you don’t end up paying tax twice.
Remember rule one — you pay tax where you earn it. So if you work in the Republic, you pay your tax there. When you then declare your worldwide income in the UK, HMRC essentially works out how much you would have paid on that income had you earned it here. If that figure is less than what you’ve already paid in the Republic, there’s nothing more to pay. If the UK would have charged you more, you only pay the difference.
The effect — unfortunately — is that you always end up paying the higher of the two jurisdictions’ tax. But for most cross-border workers, the higher jurisdiction is the Republic, so the vast majority of these UK tax returns generate no further tax charge at all.
Whilst most of these returns generate no further tax, keep in mind that the two systems don’t mirror each other perfectly. Different tax years (Ireland runs January to December; the UK, in its wisdom, runs 6th April to 5th April), different bands, different credits, and euro figures that have to become sterling ones — so there’s a bit of technical work in pulling a return together properly. And it can lead to tax payable in some scenarios: if you only worked part of the year in the Republic, say, or had other income alongside the job.
The student loan catch
If you have an outstanding UK student loan, your repayments are worked out through your tax return — and your foreign employment income counts. So you can have a return with no tax to pay, thanks to the credit relief above, and still owe student loan repayments on the same income.
I’ve had people come to me genuinely aggrieved about this, and I understand why. They expected nothing to pay, and to be fair to them, tax-wise they were right.
But here’s the way I’d frame it: those repayments are due whether you file or not. File each year and it’s a manageable annual amount that chips away at the loan. Leave it, and at some point HMRC comes looking for several years of repayments in one go — plus the stress of the brown envelope. Same money either way. One version of it is on your terms.
“I’ve been doing this for years and never filed anything”
You’re not the first, you won’t be the last, and no — you’re not going to jail.
I’d estimate a decent share of cross-border workers have never been told any of this. HMRC knows it too, and there are established routes for coming forward voluntarily and bringing things up to date. In my experience the process is calmer, quicker and cheaper than people fear — and dramatically calmer, quicker and cheaper than the version where HMRC makes the first move.
The worst plan is the one where you now know and do nothing. That’s the difference between an oversight and a decision.
A UK tax return can help with a UK mortgage
Filing because you have to is one thing. However, we have seen a number of clients find that a UK tax return gives you more options when applying for a mortgage.
UK lenders like UK paperwork. Walk in with Irish payslips and euro income and some of them get twitchy — a few will turn you down flat, not because you can’t afford the house, but because their process doesn’t know what to do with you.
A filed UK tax return puts your income in front of a lender in sterling, in a format they recognise. It doesn’t guarantee anything — no piece of paper does — but it opens doors that stay shut otherwise.
If buying a house is anywhere on your horizon, that’s reason enough to get your returns in order now, not the month you start viewing. Lenders will typically want two years of tax returns to assess affordability.
The deadlines
Your 2025/26 return — covering 6th April 2025 to 5th April 2026 — is due online by 31st January 2027. If you’ve never filed before, HMRC also expects you to register for Self Assessment by 5th October, and registration itself can take a few weeks to grind through, so don’t leave it until Christmas.
January is when everyone remembers. October is when the organised people sort it. Be an October person.
What to do about it
You’ve two options, honestly.
You can do it yourself. HMRC’s online system is there, the rules are published, and if you’ve the patience for foreign income pages and currency conversions, nobody’s stopping you. I’d never pretend otherwise.
Or you can hand it to someone who does it all day. Our team files over a hundred of these cross-border worker returns every year — it’s core work for us, not a sideline, which is why we can turn one around in days from nothing more than your payslips, and keep the fee at £250 for a straightforward case.
Either way — file. The return isn’t the enemy. It’s the thing standing between you and the brown envelope, and handled properly it might even help you buy a house.
And if you’re not sure whether any of this applies to you? Ask. It’s a two-minute answer, and it costs nothing.
