Dublin’s weather is mild and unreliable in every season, which means the calendar matters less for climate than it does for crowds and prices. There is no month you cannot visit and there are several weekends you should avoid.
The seasons
Spring, March to May. The best value and increasingly the best weather. Daylight lengthening, gardens out, and the city not yet full. May is arguably the best month of the year here.
Summer, June to August. Warmest — which means high teens to low twenties — and the longest days, with light until after ten in June. Busiest and dearest. It also rains; a Dublin summer is not a Mediterranean one.
Autumn, September to October. Mild, quieter, and the light is good. September is an excellent month.
Winter, November to February. Dark by four in December, cold rather than freezing, and by some distance the cheapest. The city is at its most itself — pubs full, no queues, and the museums empty. If you do not need daylight for photographs, late January is very good value.
Rain
Around 130 days a year with measurable rain, mostly light and frequent rather than heavy. It clears fast. Bring a waterproof jacket with a hood and shoes you do not mind getting wet, and stop planning around it.
The dates that fill the city
This matters more to your bill than the weather does.
Rugby internationals at the Aviva Stadium — the Six Nations weekends in February and March, and autumn internationals in November. These double hotel rates across the entire city and fill it.
Concerts at Croke Park and the 3Arena, which do the same on a smaller scale.
St Patrick’s Day, 17 March, and the surrounding festival. Enormous, good-humoured, and the single busiest and most expensive period. Worth doing once deliberately; a bad accident to book into by chance.
New Year and the week before Christmas.
Check the fixture and gig lists before choosing dates. It is the cheapest planning you will do.
The dates worth going for
Bloomsday, 16 June — the day Ulysses is set. Readings, walks and a lot of people in Edwardian dress eating gorgonzola sandwiches. Busy but not overwhelming.
St Patrick’s Festival, if the crowds are the point.
Dublin Theatre Festival in late September and October.
Culture Night in September, when several hundred venues across the city open free into the evening. This is genuinely excellent and almost no visitor knows about it.
Christmas, when the city is lit and the pubs are at their best, before the fortnight of peak pricing.
Daylight
The swing is severe and worth planning around. June: light from about 4:30am to after 10pm. December: light from about 8:30am to 4:15pm.
A December trip is a short-daylight trip. Museums in the afternoon, pubs after.