December in Dublin, Ireland, brings average daily highs of 9°C and overnight lows of 5°C in the 2016–2025 historical record. Warm layers deserve space in the bag. Keep longer outdoor plans easy to adjust, and allow places to stop between walks so that a colder day does not determine the whole itinerary.
In the record, at least 1 mm of rain or snow falls on 14 of 31 days on average, a daily chance of 44%. Give wet-weather alternatives a proper place in the itinerary. Arrange outdoor plans so you can move them between days, and pack a rain layer rather than relying on a completely dry run of weather.
The calculated day length, sunrise to sunset, averages about 8 hours. Sunshine averages 4 hours a day. Keep the shorter daylight window in mind when arranging longer excursions, and check local sunset times before setting out. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For a culture-focused visit, group the places you want to see into manageable areas, with time between stops. Check each venue's opening dates before fixing the order. If food is a reason for the trip, leave time around meals rather than filling every gap with another journey. Check the places you choose for current opening hours.
January has an average daily high of 8°C, 1°C lower than December. Its historical rain-day chance is 38%, compared with 44% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.