Generally, summer is best because of warmer usually) weather and plethora of summer events and festivals. Of course, it's also busy and expensive. Go in the shoulder seasons (late September-October; February to late March) for the best shot at sale airline fares and better hotel availability. Time your trip with a big event like St. Patrick's Day or the annual Six Nations rugby tournament and you're guarantee a festive time. And let's be honest, the craic in Dublin is legendary whatever the time of year, whatever the weather's like outside!