Have used Photobox a lot but find the delivery on average MUCH slower than this - up to two weeks!
I have also used www.bonusprint.ie who aren't bad - with Photobox you have to prepay credits to get the best price but with Bonusprint you can pay as you go with Visa. (Yes, you can pay as you go with Photobox as well but you don't get the lowest price.)
I have used http://www.foto.com/ireland/default.asp in the past.
Quality was very good and the delivery less than a week.
They are the cheapest I have found anyway.
I have used www.fotki.com a us website, 30 dollers yearly membership, unlimited picture storage. Picture prints work out fairly cheap with the doller being fairly weak. I have found it to be a fast service, (faster than some more local internet print services). Print quality also good.