Same place BOI. tbh travel to the US about twice a year and unless you are buying in the 1000's there is really not much of a difference from bank to bank building society to building society etc...
Give Ulster Bank a ring as they seem to have the best rate in my experience. But remember the final rate you pay for the Dollar will be the rate on the day you go in and pay for them rather than the day you order them.
GBP£ traveller's cheques worked fine for me when I visited Turkey a few years back. I guess the same would go for US$ or € equivalents. The exchange rate of something like 1.6 million Turkish Lira to the € is a bit scary though! :eek
Slightly off topic, I was there in 1990 and it was 4,000 lira to the punt which is about €3,200 to the €. I'm glad I didn't keep any liras under me matress for the next time I'd be going.
Can't remember when I was there but I do remember that the exchange rate was 15,000 TL to the IR£. I must do something about these beancounting tendancies....