Dublin is a year-round city, though summer brings the busiest crowds and the mildest weather. The old saying — "if you don't like the weather, wait five minutes" — is accurate. Bring a waterproof coat regardless of the forecast.
Summer (June–August) is the warmest and most expensive time to visit. Temperatures sit between 15–20°C. Days are long — in late June it barely gets dark before 11pm. School holiday crowds peak in July and August, and accommodation prices rise accordingly.
Spring and Autumn are the most pleasant times to visit. Cooler (5–12°C), quieter, and cheaper. The city functions normally, prices drop, and you're not competing with hen parties for pub space.
Winter (November–February) is cold and frequently wet, with January temperatures between 3–8°C. But pubs come into their own in winter — coal fires, dark evenings, unhurried conversation. Christmas in Dublin is busy; the weeks around Christmas and New Year's Eve in the Temple Bar area are raucous.
Bloomsday (16 June) is the annual celebration of James Joyce's Ulysses. Actors and readers retrace Bloom's route through the city in Edwardian costume; Davy Byrne's serves gorgonzola sandwiches and Burgundy wine. It's a genuine festival, not a tourist confection.
St Patrick's Day (17 March) transforms the city centre. The parade is large, the pubs are packed, and hotels are booked months in advance. Fun if you plan for it; chaotic if you don't.