I'd agree with Squonk, I'm a 41 here and in my Nikes I'm a US 9.5, but for other retailers i'm a 10-10.5 and 11!
I'd be wary of buying without trying the brand, is there an Irish retailler who sells the brand you want, so you can try and see?
Alternatively, forget bridal shoes and treat yourself to a good pair of designer ones, ie instead of paying €200 for white satin, pay that €200-300 for some fab ones in BT.... you'll get more wear out of them!
Or ask around and see if you can borrow someone elses shoes for the day, 'something borrowed' and all that! most people pay a huge amount for shoes that don't get to see the light of day again....