Travellers from GB will need to declare cash of €10,000 (£9,049) or more when entering Northern Ireland from 1 January.
The change is a requirement of the Brexit deal under which NI will continue to enforce EU customs rules at its ports.
HMRC says the declaration can be made online up to 72 hours in advance.
Declarations will be required for cheques and bankers’ drafts, not just notes and coins.
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Travellers from NI to GB or NI to any EU country will not need to declare cash.
Declarations are required if you’re carrying €10,000 or the equivalent in any other currency of cash and travelling to or from a non-EU country.
Northern Ireland will have a different Brexit from the rest of the UK.
The EU and UK reached a special deal on Northern Ireland in 2019.
It is called the Northern Ireland protocol and its fundamental purpose is to prevent a hardening of the border between Northern Ireland and the Republic of Ireland.
The protocol does that by keeping Northern Ireland in the EU’s single market for goods and by having Northern Ireland apply EU customs rules at its ports.
That means goods arriving from Great Britain will be checked and controlled at Northern Ireland’s ports from 1 January.
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But goods going into the Republic of Ireland and the wider EU will face no new checks or controls
‘Minimising disruption’
The Stormont Executive is due to meet virtually on Monday to discuss the post-Brexit trade deal agreed by the UK and EU last week.
Meanwhile, the Scottish government has signed a lease to use a former military airfield close to both Stranraer and Cairnryan as an emergency lorry park from 1 January.
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Up to 240 vehicles can be parked at the Castle Kennedy site if the ports go over capacity.
Traffic measures have also been introduced on a major road leading to the port at Holyhead in Wales as a means of “minimising any possible disruption to the port, town and wider community”, the Welsh government announced.
