Spring weather is very unpredictable in Ireland. Sunny and warm days are just as possible as cool, windy and rainy ones. Generally the weather is mild, April is considered quite pleasant. You’ll probably feel comfortable in a sweater and waterproof shoes. It’s the tourist mid-season and the prices aren’t as high as in summer. Spring is also the season of lambing and the best time for observing the Risso’s dolphins, the largest deep-water dolphins in the world. And yes, St.Patrick’s Day is in spring as well.