If you are VAT registered in the UK and you are purchasing goods from an Irish VAT registered person and the goods are to be dispatched to you in the UK, the supplier will take a note of your VAT number and business address and will then zero rate the supply of goods to you.
You will then account for the VAT on the goods under the reverse charge mechanism and, provided the goods you purchase are for the purpose of your trade, you will be able to claim a deduction for the VAT you have charged yourself.
Boxes 2 and 9 on your VAT return are used to record EC transactions of goods.
If arrivals from other EU Member States exceed £1,500,000 per annum you will be required to complete UK Intrastat returns.
Post Brexit the movement of goods into or out of the UK may be treated as imports and exports, meaning the reverse charge procedure will not apply and input VAT will have to be paid by the customer at the point of entry which will of course also impact cash flow. The customer may also need to consider Customs Duties and there may also be an additional administration burden on you with regards to import and export documentation.