From a weather perspective, June, July and August are the warmest and driest months to travel to the auld sod. Due to its northern latitude, it stays light out well into the evening (past 10 p.m. in June and July). So for touring, you have very long days indeed. Keep in mind, however, that Ireland, while generally mild in temperature compared to the northern reaches of the United States, can drop in temperature when the wind blows and the rain falls. Even summer temperatures can range in the 50s during the day and even down into the high 30s or low 40s at night. An Irish head wave will see temperatures rise desperately close to 80 or 85 degrees, but rarely any higher. Remember to dress in layers since Ireland gets some strong wind and weather being bordered on the West and North by the North Atlantic and North Sea.
Tour books advise traveling during the "shoulder seasons" of April and May or September and October. You'll see fewer tourist crowds and probably save a bit of budget on off season fares, but the weather tends to be a bit more questionable. The Irish Tourist Board (see Resources) is a wonderful, friendly and helpful resource.
Mid-summer, you'll see the largest crowds in and around Dublin, but even in high season, parts of Ireland along the coast can be quiet and solitary. The entire country is quite tourist-friendly with hospitality and storytelling a way of life.
The old joke goes, "My doctor told me to go to Ireland for my rheumatism. It worked. I got it in three days." So, Ireland in the winter can be a bit tough. The weather can be rainy and raw and the winds can be strong. The light is the reverse of the summer season with sundown coming at "half-three" (3:30 p.m.) as they say over there. It's a great time to visit the pubs, but a winter Irish excursion isn't for everyone.
Keep in mind also that the North coast of Northern Ireland is about the same latitude as Juneau, Alaska. Early and late in the season the northern reaches of the country can see the temperatures drop quite low.
