Farmleigh in the Phoenix Park has a lot going this month leading up to Christmas.
Every Saturday & Sunday leading up to the main event, they have a Christmas food market running from 10am to 5pm. There's storytelling, a festive crib (what would a non-festive crib include, I wonder? The one in our house always had a rhinoceros in it since my 2 year old brother decided he was a farm animal) and other kid-friendly events going each day too. Most Sundays will feature some kind of carol singers, with various different groups participating. 6 different days of activities, you've no excuse not to get down there.
Full details are here.