VAT is a 'Value Added Tax'.  In Ireland, it applies to almost everything and is already computed into the posted price.  That means that a product priced at 5 Euro (be it gasoline, groceries or a souvenir) will cost you 5 Euro -- NOT 5 Euro plus tax, but 5 Euro INCLUDING tax.

Vat Back is a tax scheme that allows you to avoid paying the Vat Tax -- but ONLY on merchandise that is leaving the country.  Unless you are planning to buy 50 Euro or more worth of gifts and souvenirs,  it's probably not worth bothering with the Vat Back paperwork.

More: https://www.shoptaxfree.com/cms/the-horizon-card/am-i-eligible/  

Note that consumables (food, gasoline, snacks), car rental, admissions and accommodation charges are NOT applicable to the Vat-Back scheme.