Although you might think it rains in Ireland all year round, it doesn't.  The rain is responsible for its "40 shades of green" but it is rarely heavy.  Most days you will have some sun, some rain and a good deal of clouds. On the east coast of Ireland, Dublin has slightly better weather than much of the country. The last two weeks in April and most of August tend to be sunny with warmer weather (12-16C in April and near 20C in August). 

A recent traveler to Ireland (late July 2011) experienced only 2 days with rain out of 9, and one of those days was a misty, foggy rain, not a downpour. 

In any case, when you are planning a trip to Dublin, make sure to bring rain gear, layers of clothes and a sense of humour about the weather - remember the weather's not why you came to Dublin! Forecast for the next 5 days.