Overall, Ireland has a mild but changeable oceanic climate with few extremes. In Ireland you may indeed experience 'four seasons in one day', so pack accordingly and keep up-to-date with the lastest weather forecast. No matter the weather, expect it to be a topic of conversation amongst the locals.
You may notice slight differences in temperate between the north and south of the country, and more rain in the west compared with the east.
Mean daily winter temperatures vary from 4°C to 7°C, and mean daily summer temperatures vary from 14.5°C to 16°C. Temperatures will rarely exceed 25°C and will rarely fall below -5°C.
Regardless of when you visit Ireland, even in middle of the summer, you will more than likely experience rain, so if you intend being outdoors, a waterproof coat is recommendedSource: Wikitravel.org