Beer and liquor are awesomely cheap. For those of you on my MSN who were treated to my 'Beerathon' updates you'll know that I drank far too much for too little. I took the world tour of breweries right from the comfort of my living room.
Grocery stores can be expensive if you're not careful. Just like here you can have the expensive (Capers, Save On, Safeway) and the cheap (Superstore, Extra Foods). A store like Dunnes can have some quality products but expect to pay for them. Tesco is your best bet. I was in there so much cashiers knew me as "That American guy" (Yeah... I know
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Clothing isn't worth it, unless you're looking for gifts or you absolutely need it. You'll pay as much as you do here ($25-30 for a decent quality top) but remember you're paying 1.5 or 1.7 of what we pay with the exchange.
Gas is pretty expensive. Fortunately most cars are fuel efficient and your trips aren't very far anyhow.
Don't go to fast food restaurants. Not only are they a rip off ($8 for a Burger King meal) but you can get much better food at a pub or restaurant for roughly the same price. Try real Irish stew or Sheppard's Pie. It'll put we have here to shame.
Your creature comforts can get pricey but you shouldn't be afraid to treat yourself.
Remember, you're on vacation and you're there to treat yourself.