I loved Ireland and went the end of June in 2011. It rained, it was sunny, it was cold, it was chilly, it was not warm only at the end in Dublin. I wore a rain jacket, long sleeves, hoodie, blazer, jeans the whole time. We were outside the whole time, as the museums are in Dublin. Of course were were in pubs too. Ireland is not like other countries where you are visiting museums, churches and indoor sites. You not doing that. There are no big cities except Dublin in the Republic of Ireland. It is all land and nature and mountains and the sea. The whole point of Ireland is being outside and seeing the land and nature and mountains and sky and sheep and sea. Going hiking and enjoying the scenery. It is a beautiful country to see in the summer, maybe late spring, early fall. But no, I would not travel in January to be outside in a country that even in the summer I would wear an Irish sweater. Others may say it is fine, but I would wait until the summer to see this beautiful country and going hiking and visit the sites. It also is very windy as you are surrounded by water when you are traveling around the coast.