Itinerary help

How many actual full days do you have there, not counting the day you arrive nor the day you depart?

Where are you flying in from - North America or someplace else far away, or just a short hop (eg London)? Makes a difference on a trip that's no more than 4 days (and may be as little as two)...

I have 3 full days not counting arrival and departure, but my arrival day is at 8am into Dublin so I'm counting that as a full day. I leave mid day on the January 1 so I agree that day I can't do much.

So you have two full days. Plus the day you arrive, exhausted and jetlagged (which I wouldn't count, no matter what time the arrival's scheduled for). With just two days, I think you're limited to one base. Stay in Dublin, and take a quickie day-trip if you want. You really don't have time for more places than that.

I agree with David, you don't have enough time to go anywhere other than Dublin as you will waste far too much time getting somewhere by public transit or car. Stay in Dublin, use your arrival day to see the city before you're too tired. Use the hop-on-hop-off bus to get around to the sights. On your second day make use of one of the many guided day trips to see some other places. In January where you go may be dependant on the weather, but Newgrange or Glendalough are popular.

You can take the DART out to Howth from Dublin fora look, not far.

While I agree with the others that just staying in Dublin and doing day trips is probably the preferred way to go for you given the limited time you have available, I would note that the train ride from Dublin down to Kilkenny only takes about an hour and a half. Would be an easy day trip or a pleasant overnighter if you decide that a visit there is preferable to staying in Dublin the whole time.
There are organized tours that will take you there as well, and which might also include a side trip to Glendalough and the Wicklow Mtns. if you prefer not do attempt it on your own.

Thank you all so much. Kilkenny seems like a good stop. Do you think it is worth spending 2 days there?

Doing it as a day trip from Dublin would probably satisfy your curiosity about the place and wouldn't eat up most of the discretionary time you have available.

With a such limited time I'd really focus on one spot. Since you would prefer not driving solo I'd stick to Dublin. A lot depends on what you enjoy most and are looking for. But from Dublin there is easy, convenient and comfortable transportation to Newgrange a 5,000 year old burial mound that is incredible. Most recommended would be a full day tour that encompasses the Wicklow Mountains and Glendalough which is one of Ireland's gems. On that tour they give you plenty of time to explore, stop at a great pub for food and rest and you get to see amazing sights. Kilkenny is nice and is reachable by tour from Dublin.

They also offer tours of the west....Cliffs of Moher, some of the ROK, Aran Islands etc......avoid those as one day tours it is LONG and in a bus a huge part of the day. Dublin itself has some fun stuff for a day. And day trips to other spots like Newgrange, Belfast, Wicklow/Glendalough will be real great locations to provide a lifetime of memories. Leave time for very friendly pubs and interacting with the locals. My daughter traveled alone and felt exceptionally safe and welcomed. Try to avoid your precious time being spent to much in transport.

Have a great trip. Ireland is SPECTACULAR !!!!!

Stay in Dublin. The the train to Howth and/or Dun Laoghaire. Plan on coming back to Ireland for a longer stay

Something to keep in mind for your trip is that the days will be short by the end of December - ie sunrise around 8:15 and sunset around 4 PM. It'll be cold and wet too, so plan (and dress) accordingly.
The "things to do" pulldown menu on TripAdvisor is pretty handy for seeing what's on offer for any of your proposed destinations. Have found a lot of little off-the-beaten-path attractions that I never knew existed by reading thru their website.