If living in Northern Ireland but fancying goods on a mainland based furniture website some of them will deliver to a mainland GB address in preparation for onward shipping.
If you explain your intention to ship the goods to the supplier they will often pack the goods appropriately for you and add additional labelling to the boxes. Of course it will depend on the individual supplier so always check the details with them and ask if they can let you know the size and weight of the chosen products.
The goods will then be your responsibility from the time they are delivered to the mainland address so bear in mind if they are damaged during shipping you cannot go back to the supplier for a refund.