Before you start

A National Insurance number
PAYE registration through your employer
A Government Gateway account, if you ever need Self Assessment
Records of any Irish or other foreign income, if you have cross-border earnings

Step-by-step

1
Check the tax code on your first payslip
PAYE deducts at source, so a wrong code means wrong deductions for months. In Northern Ireland your code should carry no nation prefix at all — an S is Scottish and a C is Welsh, and both get applied in error by payroll teams run from Great Britain who assume 'not England' means devolved rates.
Via employerWho: YouFirst payslip
2
Learn the three bands that apply here
A personal allowance, then Basic, Higher from £50,270 and Additional from £125,140 — the England and Wales structure, not Scotland's six bands. The thresholds have been frozen for several years, which quietly pulls more people into higher bands each year as wages rise.
OnlineWho: You
3
Understand the 60% trap above £100,000
The personal allowance is withdrawn by £1 for every £2 earned above £100,000, giving an effective marginal rate of about 60% between £100,000 and £125,140. Pension salary sacrifice is the standard response. It is the most consequential quirk in the system and it is signposted nowhere.
OnlineWho: You
4
Take advice before you take a cross-border job
Living in Belfast and working in Dundalk, or living in Donegal and working in Derry, or being employed by a Dublin company while sitting at a desk in Belfast, all put you into two tax and two social insurance systems. The UK–Ireland double taxation convention prevents you paying twice but does not make the paperwork go away, and getting the PAYE, PRSI and National Insurance position right from the start is enormously cheaper than unwinding it later. Frontier-worker arrangements exist and they are not automatic.
In personWho: YouBefore accepting the offer
5
Check whether you actually need to file Self Assessment
Most employees never do — PAYE handles it. You must register if you are self-employed, have significant untaxed income, or meet one of HMRC's listed triggers. Online returns are due by 31 January following the 5 April year end.
OnlineWho: You
6
Settle your residence position in year one
The Statutory Residence Test determines UK tax residence from days present and connecting factors, and the UK has replaced the old non-domiciled regime with a residence-based system. If you have Irish or other foreign income or assets, take advice in the first year rather than the third.
In personWho: You

Documents you’ll need

National Insurance number
P60 — the annual summary from your employer
P45, if you change employer during the year
Irish payslips, PPS number and Revenue correspondence, if you have cross-border income
Government Gateway credentials, for Self Assessment

Things most newcomers don’t know

Income tax is NOT devolved in Northern Ireland — unlike Scotland, and unlike the corporation tax debate you will hear about locally.

Northern Ireland uses the England and Wales bands with no separate rates and no prefix on your tax code. Scotland's S codes and Wales's C codes get applied here in error surprisingly often by payroll teams who file 'devolved nation' as one category. Check the first payslip; a wrong prefix costs real money for months.

Source: GOV.UK — income tax rates and personal allowances

The border is a tax boundary, and it is an hour away.

Belfast is the only city in this guide where a plausible job offer can put you in another country's tax and social insurance system. The UK–Ireland double taxation convention stops you paying twice, but whether you owe PAYE or Irish PAYE, whether you pay National Insurance or PRSI, and where you file, all depend on facts nobody sorts out for you. Get it right before the first payslip.

Source: HMRC — UK/Ireland double taxation convention

The 60% band between £100,000 and £125,140 is the biggest hidden rate in the system.

The personal allowance is withdrawn at £1 for every £2 above £100,000, so each extra pound in that range is taxed at the 40% headline rate plus the lost allowance — around 60% in effect. Model it before accepting an offer in that range, and look at salary sacrifice.

Source: GOV.UK — income tax rates and personal allowances

The tax year runs 6 April to 5 April, and Ireland's does not.

Almost no other country uses the UK dates, and the Republic uses the calendar year. It decides which year your arrival income falls into, when your P60 arrives, and when Self Assessment is due — and it means a cross-border worker reconciles two years that do not overlap.

Source: HMRC

Common mistakes to avoid

Not checking the PAYE code on the first payslip for a stray Scottish S or Welsh C prefix.
Assuming Northern Ireland has devolved income tax rates because Scotland does.
Taking a job across the border without settling the tax and social insurance position first.
Accepting a salary between £100,000 and £125,140 without modelling the 60% effective band.
Counting the tax year as January to December rather than 6 April to 5 April.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.