Map from Google - Dublin

The climate of Dublin 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.
Average temperatures - Dublin
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, though typically not abundant. In addition, they are well distributed throughout the year, and are frequent even in summer.
Here is the average precipitation.
Average precipitation - Dublin
Prec. (mm)655055556065557560807575760
Prec. (in)2.622.

Winter, from December to February, is quite cold: the 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; cold spells 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 temperatures rise slowly, Atlantic fronts continue to pass more or less like in the rest of the year, although they are a bit less intense than in winter. Typically the temperatures begin 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 like in the rest of the year, and with each passing week gets progressively colder. In November, the wind is often moderate or strong.

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 easier for the sky to be partly cloudy.

Cathedral of Dublin

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.
Sea temperature - Dublin
Sea (°C)87781012151515141210
Sea (°F)464545465054595959575450

When to go

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