Hi 123uk,
If the goods are sent from the EU to Northern Ireland then import VAT would not apply as it would not technically be an import but an EU dispatch.
For EU traders selling goods to individuals in Northern Ireland the supplier would charge you VAT at the rate in their country until they reach a £10K threshold which would include all of their distance sales to all EU countries including Northern Ireland.
If they have reached the £10K threshold then they should be charging UK VAT at 20%.
Please see below:
6. Supplies to customers (including private individuals) who are not registered for VAT
The onus is on the supplier to charge this VAT so we would recommend contacting them.
Thank you.