Dublin, where it's located

The climate of Dublin, capital of the Republic of Ireland, is oceanic, cool and humid throughout the year.
The average temperature goes from 5 °C (41 °F) in January and February, to 15.5 °C (60 °F) in July and August.
Here are the average temperatures.
Dublin - Average temperatures
Min (°C)2235710121210753
Max (°C)881012151820191714108
Min (°F)363637414550545450454137
Max (°F)464650545964686663575046

Precipitation amounts to 750 millimetres (30 inches) per year. The city is located in the least rainy part of the island, since a part of the rain is discharged on the west side, but the rains are frequent also in Dublin, although they are typically not abundant. In addition, they are well distributed over the seasons, and they are frequent also in summer.
Here is the average precipitation.
Dublin - Average precipitation

Winter, from December to February, is quite cold: temperatures are not so low, but wind and humidity can increase the sensation of cold. The sky is often cloudy, and rainfall is frequent. Sometimes, intense Atlantic depressions can cause wind storms.
Snowfalls are relatively rare; generally, cold spells are not intense and do not last long, and usually the temperature does not drop below -5 °C (23 °F); however, in the exceptional month of December 2010, it often snowed and the temperature dropped to -12 °C (10 °F).

In spring, from March to May, the temperature slowly rises, and Atlantic fronts continue to pass more or less as in the rest of the year, although they are a bit less intense than in winter. Typically the temperature begins to become quite mild in the second half of May.

Summer, from June to August, is cool, wet and cloudy, with quite frequent rains. Hot days are rare: the temperature may never reach 25 °C (77 °F) for an entire summer; even sunny days, or at least those in which the sky is completely clear from dawn to dusk, are quite rare.

Autumn, from September to November, is gray, cloudy and rainy as in the rest of the year, and with each passing week it gets progressively colder. In November, the wind is often moderate or strong.

Christ Church Cathedral

The amount of sunshine in Dublin is never good; however, the (relatively) sunniest season is spring. Entire days with totally clear skies are rare: it's more likely that the sky is partly cloudy.
Dublin - Sunshine

The Irish Sea is cold all year round, given that it reaches at most 15 °C (59 °F) in the summer months. Here are the average sea temperatures.
Dublin - Sea temperature
Temp (°C)87781012151515141210
Temp (°F)464545465054595959575450

When to go

The best time to visit Dublin runs from mid-May to mid-September; May and June are cooler, but they are relatively sunnier. July and August are the warmest months, or the least cool, however, it's better to bring a jacket, a sweatshirt and a sweater also in summer.