A Good Camera
You'll be doing a lot of city walking and maybe even a coastal walk or two. Make sure you have comfortable shoes. Bonus points if they're waterproof, like the Sorel Booties.
Ireland is filled with stunning landscapes and charming towns. You'll want a good camera to capture the beauty.
Ireland's weather is predictably unpredictable. But you'll likely encounter rain. Add a rain coat and umbrella to your packing list. In the winter months add warmer layers.