Officials from Revenue will be contacting 92,000 businesses around the country as preparations for Brexit continue.
Businesses will receive letters and phonecalls from officials outlining the steps necessary to prepare for Britain leaving the European Union.
The head of Revenue's Brexit Policy Unit said while businesses should register for an Economic Operators Registration and Identification number, that is only part of the process.
Lynda Slattery said business should also carry out an impact assessment to identify how Brexit can affect their company and supply chain.
She also said that companies should understand who will do their customs declarations and how import duties will affect the commodities they import.
Ms Slattery said that while officials will contact 92,000 businesses over the next eight weeks, this does not mean that all of these companies have not engaged in the Brexit preparation process.
"The 92,000 is the figure identified who had some level of trade with the UK in 2018. Some are registered, some are not. We're not taking any chances and are contacting everyone. We've written to them all and will be ringing them all,"Ms Slattery stated.
Ms Slattery also said that while there is ambiguity about Brexit, Revenue are operating on the basis that Britain will leave the EU on October 31.
She added that Revenue can provide absolute certainty that customs formalities will apply when Brexit happens.
Ms Slattery also said there is support for businesses available through the Revenue and Enterprise Ireland websites, as well as via the Department of Business.
Revenue said that based on the most up to date information available, it is estimated that 33% of businesses in the wholesale and retail trade, and 15% of businesses in the construction industry, who trade with, or perhaps buy supplies on an ad-hoc basis from, the UK have not yet applied for an EORI number.