The biggest trading partner of Northern Ireland and Scotland is the European Union. But as the UK leaves the EU, Northern Ireland and Scotland, which are part of the UK, naturally cannot continue to enjoy the overall trade dividend within the EU.
Therefore, since the BREXIT agreement between the UK and the EU, the split tendency of Northern Ireland and Scotland has been further strengthened. They hope to separate from the UK by means of an independence referendum, then sign an independence agreement with the EU as a national identity, and continue to enjoy trade policy dividends in the EU.