Read this before you start packing
Pack lightly! You will have to carry your lugagge at many points along your journey, so it is important to make sure you can handle everything by yourself.
Related to packing lightly, know the airline's luggage allowance. How many bags are you allowed to bring? How much is each bag allowed to weigh? Is there a charge if my bag is overweight or if I have an extra bag?
Luggage - Do you have luggage that will work, or is it time to go shopping? If you need to go shopping for luggage, consider the different styles of luggage (roller board, duffel, backpack) as well as what luggage needs you will have while abroad (bag for a day trip, long weekend trip, backpack for school work, etc.). Cobblestone streets do a number on roller board bags! Make sure all luggage you bring is strong and sturdy.
Weather - What is the climate of your destination? Doesn't it rain a lot in Ireland? What temperatures could you expect to have throughout your time abroad? Doing some research on the weather will help you decide what type of clothes and shoes you need. Of course you can always plan to buy clothing when you arrive as well.
Clothing - What is in style at your program location? What is culturally appropriate clothing?
Laundry - What is the laundry situation where you will be living? Will you have access to laundry whenever you want? Do you have a weekly laundry service? Knowing this will help determine how much clothes you should bring or plan to buy after arriving.