Weather in Dublin in January. The average maximum daytime temperature in Dublin in January is a cool 8°C (46°F). There are usually 2 hours of bright sunshine each day, which represents 22% of the 8 hours of daylight. The average sea temperature in and around Dublin in January is 9°C (48°F). Expect 62mm (2.4 inches) of rainfall across the whole of January with 18 days with some rain falling. The average night-time temperature is usually a cold 3°C (37°F).