As of 1 January 2021 (at the end of the transition period) the UK will leave both the Customs Union and the Single Market. With or without an agreement between the UK and the EU, many significant changes will then take place in relation to GB trading with Northern Ireland, notably in relation to customs and border procedures. These are dealt with in the Northern Ireland Protocol (‘the Protocol’). The Protocol is a practical solution to avoid a hard border with Ireland whilst ensuring the UK, including Northern Ireland, leaves the EU as a whole, enabling the entire UK to benefit from future Free Trade Agreements (FTAs).
There will be special provisions which apply only in Northern Ireland while the Protocol is in force. These are currently the subject of separate discussions under the Joint Committee procedure established by the Protocol. A large number of practical issues relating to trade between GB and NI remain unresolved.
However goods will not need to be checked when crossing the border in to the Republic of Ireland. Latest government information can be found here: Moving Goods under the Northern Ireland Protocol though details may change before the end of the year.
A summary of these arrangements was set out in the House of Lords on 26 October 2020 by Defra Minister Lord Goldsmith here.
In Northern Ireland The Department of Agriculture, Environment and Rural Affairs contact details to help with all queries on imports of composite products into NI can be found : https://www.daera-ni.gov.uk/contacts/daera-helpline-numbers.