Rain Gear
It rains a lot in Ireland, as seen by the wealth of greenery that blankets the nation even in the summer. This is absolutely one of the things to know before traveling to Ireland. You don't want to be caught without your rain gear because the weather might be unexpected. Of course, once you're in Ireland, you can always buy seasonal goods.
However, there is so much to see and do in this lovely nation that you don't want to waste time shopping for insignificant stuff like umbrellas. You're more inclined to splurge on an umbrella if you need one quickly. Always carry a rain gear and a small umbrella while traveling to Ireland in the summer. You've probably looked out the window and thought to yourself. It didn't last long, but it was sporadic, so you had to be ready.
You may even slip your little umbrella into the bottom of your bag and leave it there for the duration of your journey. Depending on your activity or clothing choice, an umbrella could be preferable than a coat, or you might combine the two.