You'll need a bank account in the US in any case. I'd set up an account there. Your problem is exchange rates and the high costs associated with changing money. You would be hit for this each time you use your current account card etc.
But it is a good time to buy dollars so a lump sum transfer might be a good idea, shop around to get the best deal.
Also where I am we recently received a letter from our bank which says we cannot withdraw money in the US from our current account, some kind of new safety measure, not sure if it applies to Irish accounts. My OH is in the US today so I'll ask him to see if he has withdrawn any money but I know he didn't the last trip.