They add "Import Fees Deposit" which, at first glance, seems quite substantial but the product price is reduced from the initial price (UK VAT inclusive) displayed. To illustrate, for the CO-9D the price displayed on the product page is £18.59 and invoice/receipt shows Items priced at £15.82 and Import Fees Deposit at £4.58 giving a total of £20.40.
If you take the UK VAT (17.5%) off the £18.59 you get £15.82 so that's correct. However, adding Irish VAT @ 21% would be £3.32. As far as I can ascertain they are trousering the remaining £1.26. Default (priority or whatever it's called) P&P is priced at £6.00 so it seems that they add Irish VAT to that (21% of £6.00 is precisely £1.26) but don't take it back off when you select non-priority Free delivery. Notably it's described as a deposit so I guess one is entitled to reclaim any inapplicable portion?
That was a very good question 'erm, without doubt the best question you have ever asked since you first got connected to the world wide web. I wouldn't have done the mathematics and hence wouldn't have noticed the VAT on non-existent delivery fees had you not asked.
However, the bottom line is still a bit less than Screwfix charge for the same item in the Republic of Ireland. Paying in Sterling - never accept Amazon's automatic exchange rate, always select Her Majesty's Shilling at checkout or your being robbed - via Revolut worked out at €23.50 whereas it would have cost €26.95 in Screwfix ROI or £22.99 (€26.24 @ current Revolut exchange rate) in Screwfix UK.