Temperatures
Temperatures will rarely go below 32°F and only occasionally above 68°F - with June, July and August being the warmest months, January and February the coldest. Extremes are not unknown though. The summer of 2006 was the hottest on record for ages. On the other hand rare freezing spells tend to bring the country to a grinding halt and even a sprinkling of snow will have most drivers panicking.
Clothing
The secret of coping with the Irish weather lies in taking the correct clothing. You should prepare for moderately mild weather at all time and be able to supplement basic dress with a warm sweater and/or rainproof top. A hat is a good idea, an umbrella is not. Be careful on sunny days especially on the beaches, the breeze can cool you while the sun still burns your skin. And take sensible footwear, most rural attractions and even some urban areas are best described as "rugged terrain" and the ground is bound to be wet occasionally.
Best Time to Travel
When should you go to Ireland then? It all depends on your interests. Outdoor fun is only for the hardiest from mid-November to February. Otherwise you could really pick any time for a holiday, with substantial savings to be had outside the tourist season.
The tourist season in Ireland generally runs from Easter to the October Bank Holiday, outside these times attractions as well as accommodations might be closed. Check carefully when planning your trip outside the tourist season - though the peace and quiet even at major attractions can be rewarding.