What a wonderful opportunity! Ireland is such a beautiful country with friendly welcoming people.
I wouldn’t stress that much over clothes. You will be able to do laundry so take about a weeks worth of bottoms, tops and underwear. Most important for me would be taking comfortable shoes that I love. Wouldn’t want to have to buy and break in new shoes.
Plan to buy soap/toothpaste/shampoo once you get there. Take a small amount to get started then buy more.
The electric outlets are different so you will need plug adapters so you can plug things in. The electric current is also different so you will need to figure out if any of your electronics need electric current conversion. If you need a hairdryer or anything that heats up, buy it after you arrive.
How will you pay for things? Most places you can tap to pay. Do you have a credit or debit card that doesn’t charge foreign transaction fees? If you don’t I would recommend Capital One.
As others have said figure out your phone options before you leave the US. Is your phone locked or unlocked? What international plans does your US carrier offer?
If you have more questions come back and ask so we can help make your trip go smoothly.