Map from Google - Ireland

The island of Ireland has an oceanic climate, cool and damp, cloudy and rainy throughout the year. Both the diurnal and the annual temperature range are narrow, so both the summer heat and the winter frost are rare.
Here are the average temperatures of Dublin, the capital city, located on the east coast.
Average temperatures - Dublin
DublinJanFebMarAprMayJunJulAugSepOctNovDec
DublinJFMAMJJASOND
Min (°C)2235710121210753
Max (°C)881012151820191714108
Min (°F)363637414550545450454137
Max (°F)464650545964686663575046

In Kerry, located on the southwest coast and more exposed to Atlantic currents, temperatures are slightly higher in winter and lower in summer. Here are the average temperatures.
Average temperatures - Kerry
KerryJanFebMarAprMayJunJulAugSepOctNovDec
KerryJFMAMJJASOND
Min (°C)5556811131311975
Max (°C)101011131517181817141210
Min (°F)414141434652555552484541
Max (°F)505052555963646463575450

In Ireland, Atlantic weather fronts move relentlessly one after another over the country, resulting in a rapid succession of cloudiness and sunshine, rain or showers and subsequent improvements. Days with completely clear skies are rare: the weather is more likely variable or unstable, and in the interludes between disturbances you can expect, more than clear skies, banks of clouds running in the sky.

The wind in Ireland is frequent and lively, although it is generally stronger between late autumn and early spring. The windiest areas are the northern ones, which are literally windswept; the (relatively) less windy are the inland south-eastern areas (see Thurles, Kilkenny).

Rainfall is frequent throughout the island, but especially along the west coast, where it rains on average even more than once every two days; rainfall is more abundant on the slopes of the western hills, where it exceeds 2,000 millimetres (80 inches) per year. The least rainy area is the eastern one, where Dublin is located: here the rainfall is around 760 mm (30 in) per year.
Here is the average precipitation in Dublin.
Average precipitation - Dublin
DublinJanFebMarAprMayJunJulAugSepOctNovDecYear
DublinJFMAMJJASONDY
Prec. (mm)655055556065557560807575760
Prec.(in)2.622.22.22.42.62.232.43.13329.9
Days131011111110111210121213135

In Cork, on the south coast, 1,200 mm (48 in) of rain per year fall. Here is the average precipitation.
Average precipitation - Cork
CorkJanFebMarAprMayJunJulAugSepOctNovDecYear
CorkJFMAMJJASONDY
Prec. (mm)1301001007580808095951401201351230
Prec.(in)5.13.93.933.13.13.13.73.75.54.75.348.4
Days211720161615161616201920212

Winter in Ireland, from December to February, is quite cold but not freezing. The sky is often cloudy, the rains are frequent, and the most intense low pressure areas can cause wind storms. Night temperatures hover on average around 1/2 °C (34/36 °F) in inland areas, and around 2/3 °C (36/37 °F) along the coasts, while highs range from 6/7 °C (43/45 °F) in inland areas, to 7/8 °C (45/46 °F) along the coasts. The south-west coast (see Kerry, Waterville and Cahirciveen, and the island of Valentia) has a particularly mild climate, in fact here the average temperature in the coldest months goes from 5 °C (41 °F) at night, to 10 °C (50 °F) during the day (as we have already seen in the table regarding the temperatures of Kerry), and frosts are virtually absent, so that even sub-tropical plant species can grow there.
On milder periods, when southern air masses prevail, the temperature in Ireland can reach 15 °C (59 °F) even in winter.
Cold waves are rare, and are usually short-lived, because after a while the westerlies start to blow again. Even snow is quite rare: on average, it falls for a few days in a year, and may not occur for an entire winter. Night temperatures drop below freezing (0 °C or 32 °F), usually by a few degrees, for about twenty days each year in Dublin, but only in the coldest winters they can drop to about -10 °C (14 °F). In most cases, during clear nights there can be light frosts, and nothing more.
In Ireland there are no major mountain ranges, however, in the hills south of Dublin (Wicklow Mountains) and more rarely in the western highlands, with reach about a thousand metres (3,300 feet) in height, during winter the rain can turn into snow above 400/500 metres (1,300/1,600 feet).

Spring, from March to May, is initially cold, and remains very cool or cold still in April, and sometimes in May; usually the temperature becomes quite mild only from the second half of May, when, however, nights can still be quite cold. In return, spring is the (relatively) least rainy season and the sunniest of the year.

In Summer, from June to August, temperatures are cool: average highs are around 16/17 °C (61/63 °F) in Ulster and northern areas, 17/18 °C (63/64 °F) in the centre, and 18/19 °C (64/66 °F) in the south, while lows swing around 11/12 °C (52/54 °F) everywhere. The temperature rarely exceeds 25 °C (77 °F), and this happens only in the rare and short periods when the Azores High moves over the country, while it almost never reaches 30 °C (86 °F).

Autumn, from September to November, is cloudy and rainy, at times windy, with little room for sunshine.

Ireland

The sea does not encourage you to swim, because the water temperature reaches only 15 °C (59 °F) in July and August.
Sea temperature - Dublin
DublinJanFebMarAprMayJunJulAugSepOctNovDec
DublinJFMAMJJASOND
Sea (°C)87781012151515141210
Sea (°F)464545465054595959575450

When to go

The best time to visit Ireland is from mid-May to August: the rains are frequent, the air is often very cool (especially in May), it's better to bring a jacket and umbrella (or raincoat as it often rains in the wind), but the days are long, and there's some hope for the sun to peek through the clouds. May and June are cooler than July and August, but on the other hand they are (relatively) sunnier, and you can enjoy the nature in bloom.
In September, temperatures are still acceptable, but the sky is often cloudy, and the days are shorter than in the previous months.

What to pack

In winter: warm clothes, sweater, coat, raincoat.
In summer: clothes for spring and autumn, jacket and sweater, raincoat or umbrella.