Hopefully your "4 to 5 days" is 4 to 5 days there, on the ground, not counting your arrival or departure days. Most flights from the US leave late in the day, fly overnight and arrive in Ireland in the morning (ie a "redeye" flight). Note that your arrival day will typically be in a jet-lagged fog, so don't count that as usable time. Likewise, on your departure day, no matter what time your flight is scheduled, that whole day is going to be consumed with tasks around getting to your return flight.
Are you really planning to go all the way to Europe for just 4 days or so? If that's all you can manage, and that's all you want, OK, but...you know it's a long way to go for what effectively works out as a long weekend. The cost of your airfare will be better spread over a longer stay if you can swing it. There's plenty to see and do in Ireland, to me it seems a shame to go all that way and only be there for a few days.
Your bank will definitely charge you a fee for exchanging USD for Euro, but it's not a bad idea to arrive with at least some cash
You really do not need to (and don't want to) "exchange" any money. Just bring your ATM/debit card linked to your bank at home. Upon arrival, you will find plenty of perfectly functional ATMs in your arrival airport. You can't miss them. Just use your ATM/debit card in one of those machines to pull some cash (in Euros) from your account at home. Yes, give your bank(s) a heads up so they don't suspect fraud and lock your card. Many of us always travel with at least 2 ATM cards and 2 credit cards (each linked to separate accounts...avoid having all your eggs in the same basket).
One last consideration: however many days you do stay, in February, the days are going to be on the short side, so be sure to factor that into your plans (and how much you plan to accomplish each day).
Ireland's great. Have fun.