Firms here have been warned to mitigate against risks to their business as trade talks between the UK and EU near their conclusion.
Bank of Ireland said it was urging companies to consider their cashflow, currency and customs.
The bank said new arrangements were likely to bring increased costs. "For example, changes to business processes related to customs compliance and changes in the supply chain could result in increased working capital requirements," it said.
And an increase in exchange rate volatility is another "potentially significant" impact, the bank added. Customs issues to be taken care of included availing of trader support services, supplier management, trade tariff codes and getting an Economic Operators' Registration and Identification (EORI) number.
The bank's Niall Devlin said: "We truly hope for a deal, but whatever the outcome, how NI businesses operate will change from January 1...
"NI businesses that trade with the EU and GB need to be ready from a cashflow, currency and customs perspective and there are steps to take in readiness."
Trade between NI and Great Britain is addressed in the NI Protocol, devised to avoid a hard border on the island.
But a report by the Department for the Economy (DfE) has said the protocol would damage the economy here, putting up to 23,000 jobs at risk in the long-term. It was written before the publication of rules on the protocol and their approval by the EU-UK joint committee.
A DfE spokesperson said the detail would have a limited bearing but would "narrow the range of the results" described in the report.
"However, it is anticipated that, in the absence of enduring measures and mitigations aimed at sustaining the strength of trading links between NI and its largest trading partner, GB, the risk remains that the key economic impacts set out in the report's conclusion would be realised."