Hi, from 2021 European governments are making organisations like Ebay responsible for charging and collecting VAT on sales through their portals.
So an Irish seller will have UK VAT applied to any sales into the UK. I believe something similar already happens with sales into the USA, US sales tax gets applied whether you like it or not.
The Irish seller needs to do nothing. Ebay adds VAT to the selling price, the buyer pays the VAT to Ebay then Ebay pays the VAT to the appropriate government.