€2.45 cheaper in United Kingdom than in Ireland, before delivery.
Buying from another country is not always the bargain it looks. Check that the shop delivers to you, what delivery costs, how long it takes, and whether customs or VAT is added on arrival. Inside the EU there is normally no extra duty; from outside, there often is.
Buy the exact brand and model your optician prescribed. Lenses with the same power can differ in material, base curve and diameter.