Ahh, the weather. If I could predict that I would be a millionaire! Generally June is just a bit warmer than September. (See the temperature chart on my "Weather" page.) You can get good weather or horrible weather at any time. With weather we are always speaking in generalities. So generally, September is one of the drier months. Then again you could hit a spell of rainy weather. When going to Ireland you just pack the appropriate clothes and wish for the best!