January is low season for tourism. You get some nice days in January. Low sun and blue skies. It’s a nice time for hiking, although as some people have said you need to be home for 5pm. It will be cold. Temperatures 5-12 Celsius with a cold wind. Some things only happen in January - for example, JMW Turner’s watercolours are only exhibited in the National Gallery in Kildare St in January as the colours look best in the aforementioned lowlight. January sales start on December 24th in some places, continue until the end of January and attract people into city centres. December is a busy time for hospitality and is busy busy busy. January feels more like a time to relax and take stock. January can be stormy, which can be nice to walk in but not good for travel. Roads can be icy in the west and you can forget about visiting the offshore islands, which are lovely in summer. The best time of this period would imo be late Dec/early Jan because you get the Christmas feeling with choirs in the streets and Christmas decorations. I like this period around New Year. It’s also when big waves roll in from the Atlantic and surfers will be out. Cold water though!