The countryside in early December? Risky. Short days, possibility of cloud cover and/or rain.

I'd suggest a city break but with the possibility of an excursion or two if the weather is propitious.

As Padraig said, the countryside in December may be risky. I'd suggest you could spend a couple of days in Dublin since there are plenty of things to see and the city will look amazing all decorated with Christmas lights. Do not miss the Trinity College and Book of Kells, the National Museum and Gallery, Phoenix Park and the Guinness Storehouse, having a drink at the Gravity bar with the city at your feet is an unforgettable experience. Take a stroll in the buzzing area of Temple Bar and enjoy some traditional music.
If you want to explore the countryside and see some breathtaking scenery, I'd recommend you to take a day tour in County Wicklow, drive through the National Park, visit the beautiful Powerscourt Gardens and one of the oldest monastic sites of Ireland: Glendalough.
If you want to spend the night in the area, the Brook Lodge Hotel in Maccredin Village is a great place to stay, right in the middle of nowhere, in a timeless atmosphere. From Dublin you can also easily reach Northern Ireland, either with a car or with public transports. You could spend one day in the Antrim Coast and visit the wonders on the coastal route like the Giant's Causeway, the Carrick-a-rede bridge, Dunluce Castle and the Mussenden Temple.
If you have to chose between hiring a car or taking public transport, I'd suggest you to hire a car so that you can reach the hidden magic corners. Alternatively you can take day escorted tour from Dublin. Enjoy your adventure in Ireland!