Ireland's weather is maritime — mild, wet, and changeable year round. There is no bad time to visit, though some seasons are clearly easier than others.
May to September is when most visitors come, and for good reason. Temperatures in summer range from about 15°C to 20°C (59–68°F), occasionally warmer. June and July bring the longest days: sunset in the far west can be after 10pm. Rain is possible at any time but summer showers tend to be brief. Accommodation costs rise in July and August, and popular sites like the Cliffs of Moher and the Ring of Kerry get busy. Booking ahead is advisable.
April and October can both be rewarding. Crowds are lighter, prices drop, and the landscape is beautiful — spring green in April, russet and gold in October. Weather is cooler and less predictable, with more chance of sustained rain.
November to March is the quiet season. Many B&Bs and some smaller attractions close or reduce hours. The west coast can be genuinely rough in winter, with Atlantic storms rolling in regularly. But the cities — Dublin, Cork, Galway — are lively year round, prices are low, and you will have ancient sites like Newgrange and the Rock of Cashel largely to yourself. The winter solstice alignment at Newgrange is one of the country's most spectacular annual events.
The one reliable thing about Irish weather is its unreliability. A trip in March can produce crisp sunny days; a week in August can be cold and grey. Always pack a waterproof jacket. Layering is more useful than heavy coats.