Prices are not greatly different than you might find around the U.S. but like the U.S. or any other country they can vary greatly from area to area. You will pay higher prices across the board in Dublin than other cities and prices in rual areas may be even lower. It can also get confusing dealing with a new currency. With the change over to the Euro there is some built in confusion taking place in Ireland so you won't have to worry about holding things up while you check the change carefully to get the amount right.
The biggest problem you will encounter is the natural urge to try and mentally convert every transaction into your home currency. I spent my first 2 years in Ireland mentally converting everything from pounds to dollars and from dollars to pounds. Once I gave that up and thought of the money I had on hand as pounds and what I spent in pounds life got much easier. Now we are all going through this process again as we change from pounds to Euro! You will possibly find many items priced in pounds (£) or Euro (?) during the change over adjustment period here but the Euro price is what you will be paying.
Items in Ireland have what is called value added tax, or VAT. As of March it will be 21%. While similar to sales tax in the states, it is already added into the posted price of an item so you pay what ever the price tag reads. I found that many prices, even with this 21% tax added in, come to the same price as what I paid in the states, such as at McDonalds or Burger King, and some come out cheaper. Of course some things will be higher but I haven't noticed it to a great extent. If you live outside the European Union be sure to save all your receipts since you can get the VAT you paid refunded at Shannon or Dublin airports or you can mail in the form after you return home.
As anywhere you will get better prices in the small local shops in a town or city as oppossed to the 'tourists' shops. I know of one location where the tour bus drivers encourage visitors to shop in the facility where they have parked only for them to discover the items they bought there available at up to half the price in the nearby village shops.
Most of the prices listed below are ones I know in the Cork area. Keep in mind that after 5 years I know where to shop to get the best prices so some prices are what I know to be the lower end and I have put a + sign after the price to indicate this.