As another poster asked, why would you buy dollars now if you expect them to fall. With the fed expected to drop interest rates further, i can only really see the dollar losing further traction against the euro, which for the average european holiday maker like yourself is great news. Even if the dollar does claw back a bit, I wouldn't think that it's enough to bother worrying about for the sake of some holiday spending money (i.e. <5k)
With regards to the taking money out now/using ATM's over there... even though alot of Irish banks will call it "free", american banks charge a withdrawl fee of around $2 at most ATMs. Personally, I brought over $300 cash (didn't feel safe bringing more) and withdrew cash $200 at a time when I was over there so that my transaction fees were only about 1-2%.
In essence, with the strenght of the euro, no matter what you do it will be relatively cheap! Spend less time worrying about small fluctuations that might occur and more time enjoying your holiday!